refs: make delete_refs() virtual

refs: make delete_refs() virtual

In the file-based backend, delete_refs has some special optimization
to deal with packed refs.  In other backends, we might be able to make
ref deletion faster by putting all deletions into a single
transaction.  So we need a special backend function for this.

Signed-off-by: David Turner <redacted>
Signed-off-by: Junio C Hamano <redacted>
Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>