refs/files: remove unused "oid" in lock_ref_oid_basic()

refs/files: remove unused "oid" in lock_ref_oid_basic()

In the preceding commit the last caller that passed a non-NULL OID was
changed to pass NULL to lock_ref_oid_basic(). As noted in preceding
commits use of this API has been going away (we should use ref
transactions, or lock_raw_ref()), so we're unlikely to gain new
callers that want to pass the "oid".

So let's remove it, doing so means we can remove the "mustexist"
condition, and therefore anything except the "flags =
RESOLVE_REF_NO_RECURSE" case.

Furthermore, since the verify_lock() function we called did most of
its work when the "oid" was passed (as "old_oid") we can inline the
trivial part of it that remains in its only remaining caller. Without
a NULL "oid" passed it was equivalent to calling refs_read_ref_full()
followed by oidclr().

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>