repack, prune: drop GIT_REF_PARANOIA settings

repack, prune: drop GIT_REF_PARANOIA settings

Now that GIT_REF_PARANOIA is the default, we don't need to selectively
enable it for destructive operations. In fact, it's harmful to do so,
because it overrides any GIT_REF_PARANOIA=0 setting that the user may
have provided (because they're trying to work around some corruption).

With these uses gone, we can further clean up the ref_paranoia global,
and make it a static variable inside the refs code.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>