Remove redundant whatwg import

This has been rewritten to use browser-request rather than whatwg, so remove
the import, which was causing the build to fail on npm 2. (It worked, for the
wrong reasons, on npm 3.)

src/components/views/dialogs/ChangelogDialog.js

@@ -16,7 +16,6 @@
 
 import React from 'react';
 import sdk from 'matrix-react-sdk';
-import 'whatwg-fetch';
 import request from 'browser-request';
 
 const REPOS = ['vector-im/vector-web', 'matrix-org/matrix-react-sdk', 'matrix-org/matrix-js-sdk'];