Remove Tusky's Build

We're about to release the full version of the app tomorrow. But the new version is dependent on the Firebase library & service for push notifications. To my understanding, this is incompatible with F-Droid's build system since it's a google play service.

So, Tusky needs to have its build disabled at least. It'd probably be best to remove from the repository. We aren't going to have a version without Firebase soon and having this version get stale in your repo seems senseless.