Clean up obsolete oauth entrypoint #1038
Description
/oauth/callback.php is a left-over from the pre-newinternal days that was kept around so we didn't have to recreate the OAuth application on-wiki and force everyone to re-authenticate.
Since we enabled OAuth-based creation, we had to create a new OAuth application and force everyone to re-authenticate anyway, so production is now using the /internal.php/oauth/callback/authorise path instead.
We should delete /oauth/callback.php and anything that becomes unused due to it's deletion (OAuthRequestRouter comes to mind, but it may not be the only thing).