Remove left panel widgets (#8044)

They aren't practically used, and are difficult to set up in the first place (requires server-managed account or devtools).

src/settings/Settings.tsx

@@ -882,10 +882,6 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         supportedLevels: LEVELS_ROOM_OR_ACCOUNT,
         default: {},
     },
-    "Widgets.leftPanel": {
-        supportedLevels: LEVELS_ACCOUNT_SETTINGS,
-        default: null,
-    },
     "Spaces.allRoomsInHome": {
         displayName: _td("Show all rooms in Home"),
         description: _td("All rooms you're in will appear in Home."),