server: rely on rsync to delete files from git-mirror
server: rely on rsync to delete files from git-mirror

Instead of each time deleting the whole local git-mirror repo, and
recreating it, this just deletes the .git/ dir, then lets the rsync in
_local_sync() handle deleting anything that should no longer be in the repo.

f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver