Fix composer.json to prevent infinite loops.

So it turns out you're not supposed to specify the repository in
composer.json, as that's only for dependencies. Otherwise it will loop
infinitely.

The repository tag isn't for the current application, it's for
dependencies of the application. I've tried doing "composer install"
with MediaWiki master and it loops infinitely because it goes to the
repository, sees that it has a composer.json, reads it, sees that it has
a repository, etc.

This patch thus Remove the 'repositories' statement pointing to
Wikimedia gerrit installation which as added with 0ee7f355

Change-Id: I9a77d28f33a0b966f42c8c2e952b922809acc20d