remove outdated internal messages app. New one is in app store, fix #987

So @kadukeitor is the original author of the internal messages app and rightly complained about a modified version by @TharinduDG being in this repository here.

The original version is at https://github.com/kadukeitor/internal_messages and published as an ownCloud app: http://apps.owncloud.com/content/show.php/Internal+Messages?content=154784

Please review @karlitschek @DeepDiver1975 @kadukeitor