Mozilla Sync is developed in its own repo now

Remove Mozilla Sync app as it is developed in its [own
repository](https://github.com/owncloud/mozilla_sync/) now.

See also the [Maintainers wiki
page](https://github.com/owncloud/core/wiki/Maintainers).
Merge pull request #1454 from ogasser/mozilla_sync_new_repo

Mozilla Sync is developed in its own repo now