Remove unused translations

```
F:\Workspaces\riot-web\riot-web>yarn prunei18n
yarn run v1.19.1
$ matrix-prune-i18n
ar.json: removed 20 translations
az.json: removed 1 translations
bg.json: removed 29 translations
bn_IN.json: removed 1 translations
ca.json: removed 28 translations
cs.json: removed 28 translations
da.json: removed 27 translations
de_DE.json: removed 29 translations
el.json: removed 27 translations
en_US.json: removed 28 translations
eo.json: removed 28 translations
es.json: removed 28 translations
eu.json: removed 28 translations
fa.json: removed 26 translations
fi.json: removed 27 translations
fr.json: removed 29 translations
ga.json: removed 3 translations
gl.json: removed 28 translations
he.json: removed 26 translations
hi.json: removed 28 translations
hr.json: removed 27 translations
hu.json: removed 29 translations
id.json: removed 27 translations
is.json: removed 27 translations
it.json: removed 29 translations
ja.json: removed 27 translations
jbo.json: removed 27 translations
ka.json: removed 27 translations
ko.json: removed 28 translations
lt.json: removed 27 translations
lv.json: removed 28 translations
ml.json: removed 25 translations
nb_NO.json: removed 16 translations
nl.json: removed 29 translations
nn.json: removed 28 translations
pl.json: removed 28 translations
pt.json: removed 27 translations
pt_BR.json: removed 28 translations
ro.json: removed 28 translations
ru.json: removed 29 translations
sk.json: removed 28 translations
sl.json: removed 3 translations
sq.json: removed 29 translations
sr.json: removed 28 translations
sr_Latn.json: removed 1 translations
sv.json: removed 28 translations
ta.json: removed 7 translations
te.json: removed 2 translations
th.json: removed 25 translations
tr.json: removed 28 translations
uk.json: removed 28 translations
vi.json: removed 9 translations
vls.json: removed 2 translations
zh_Hans.json: removed 28 translations
zh_Hant.json: removed 29 translations
Done in 0.19s.
```

Fixes https://github.com/vector-im/riot-web/issues/11539

src/i18n/strings/ja.json

@@ -1,41 +1,14 @@
 {
-    "Riot is not supported on mobile web. Install the app?": "Riotはスマートフォンでの表示に対応していません。できればアプリをインストールして頂けませんでしょうか?",
     "Welcome to Riot.im": "Riot.imへようこそ",
-    "Search the room directory": "部屋一覧を検索",
     "Unknown device": "不明な端末",
     "%(appName)s via %(browserName)s on %(osName)s": "%(appName)sは%(osName)sの%(browserName)s上で動作しています",
     "Custom Server Options": "カスタムサーバのオプション",
-    "You can use the custom server options to sign into other Matrix servers by specifying a different Home server URL.<br/>This allows you to use Riot with an existing Matrix account on a different home server.<br/><br/>You can also set a custom identity server but you won't be able to invite users by email address, or be invited by email address yourself.": "他のホームサーバのURLを指定することで、他のMatrixサーバにサインインするためにカスタムサーバのオプションを利用できます。<br/>これにより、他のホームサーバ上で既存のMatrixアカウントを用いてRiotを利用できます。<br/><br/>カスタムアイデンティティサーバを立てることもできますが、メールアドレスでユーザを招待する、または自身が招待されることはできません。",
     "Dismiss": "やめる",
     "powered by Matrix": "powered by Matrix",
     "Riot Desktop on %(platformName)s": "%(platformName)s向けデスクトップ版Riot",
     "You need to be using HTTPS to place a screen-sharing call.": "画面共有通話を行うにはHTTPS通信を使う必要があります。",
     "Decentralised, encrypted chat &amp; collaboration powered by [matrix]": "[matrix] による、分散型で暗号化された会話とコラボレーション",
-    "Lots of rooms already exist in Matrix, linked to existing networks (Slack, IRC, Gitter etc) or independent. Check out the directory!": "既にたくさんの部屋が、既存のネットワーク（Slack、IRC、Gitter等）に接続して、または独立的にMatrixに存在します。ディレクトリを検索してください！",
     "Chat with Riot Bot": "Riot Botと会話",
-    "Get started with some tips from Riot Bot!": "Riot Botにヒントをもらって始めましょう！",
-    "General discussion about Matrix and Riot": "MatrixとRiotの概略",
-    "Discussion of all things Matrix!": "Matrixなんでも議論！",
-    "Riot/Web &amp; Desktop chat": "Riot/Web &amp; デスクトップ版チャット",
-    "Riot/iOS &amp; matrix-ios-sdk chat": "Riot/iOS &amp; matrix-ios-sdk チャット",
-    "Riot/Android &amp; matrix-android-sdk chat": "Riot/Android &amp; matrix-android-sdk チャット",
-    "Matrix technical discussions": "Matrixについての技術的な討論",
-    "Running Matrix services": "Matrixサービスを実行しています",
-    "Community-run support for Synapse": "コミュニティ運営のSynapseのサポート",
-    "Admin support for Dendrite": "Dendriteの管理者サポート",
-    "Announcements about Synapse releases": "Synapseの公開についてのお知らせ",
-    "Support for those using and running matrix-appservice-irc": "matrix-appservice-ircを利用並びに運営している方へのサポート",
-    "Building services on Matrix": "Matrix上でのサービスの開発",
-    "Support for those using the Matrix spec": "Matrixスペックを利用する方へのサポート",
-    "Design and implementation of E2E in Matrix": "Matrixのデザインとエンドツーエンドの実装",
-    "Implementing VR services with Matrix": "MatrixでのVRサービスの実装",
-    "Implementing VoIP services with Matrix": "MatrixでのIP電話サービスの実装",
-    "Discussion of the Identity Service API": "Identity Service APIの議論",
-    "Support for those using, running and writing other bridges": "他のブリッジを利用、運営、作成している方へのサポート",
-    "Contributing code to Matrix and Riot": "MatrixとRiotにコードを提供する",
-    "Dev chat for the Riot/Web dev team": "Riot/Web開発者チームのための開発者チャット",
-    "Dev chat for the Dendrite dev team": "Dendrite開発者チームのための開発者チャット",
-    "Co-ordination for Riot translators": "Riot 翻訳者による共同作業",
     "Unexpected error preparing the app. See console for details.": "アプリケーションの準備中に予期しないエラーが発生しました。詳細はコンソールを参照してください。",
     "Your Riot is misconfigured": "あなたのRiotは設定が間違っています",
     "Invalid configuration: no default server specified.": "不正な設定です：デフォルトのサーバーが設定されていません。",