refs.c: make delete_ref use a transaction

refs.c: make delete_ref use a transaction

Change delete_ref to use a ref transaction for the deletion. At the same time
since we no longer have any callers of repack_without_ref we can now delete
this function.

Change delete_ref to return 0 on success and 1 on failure instead of the
previous 0 on success either 1 or -1 on failure.

Signed-off-by: Ronnie Sahlberg <redacted>
Reviewed-by: Michael Haggerty <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>