Previously everything from a repo staying inside the db when removing it
without disabling it first, the problem manifests when the repo is
readded later (or a mirror), as it would get a new id but all apk
entries still point to the original repoid.
So we now first disable a repo (which just calls
RepoProvider.Helper.purgeApps) before deleting it from the db.
closesBubu/fdroidclassic#29
a965deb51e