refs: implement removal of ref storages

We're about to introduce logic to migrate ref storages. One part of the
migration will be to delete the files that are part of the old ref
storage format. We don't yet have a way to delete such data generically
across ref backends though.

Implement a new `delete` callback and expose it via a new
`ref_storage_delete()` function.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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