refs/files: remove unused "extras/skip" in lock_ref_oid_basic()

refs/files: remove unused "extras/skip" in lock_ref_oid_basic()

The lock_ref_oid_basic() function has gradually been replaced by use
of the file transaction API, there are only 4 remaining callers of
it.

None of those callers pass non-NULL "extras" and "skip" parameters,
the last such caller went away in 92b1551b1d4 (refs: resolve symbolic
refs first, 2016-04-25), so let's remove the parameters.

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