reftable/fsck: use REFTABLE_UNUSED instead of UNUSED

reftable/fsck: use REFTABLE_UNUSED instead of UNUSED

While we have the reftable-specific `REFTABLE_UNUSED` header, we
accidentally introduced a new usage of the Git-specific `UNUSED` header
into the reftable library in 9051638519 (reftable: add code to
facilitate consistency checks, 2025-10-07).

Convert the site to use `REFTABLE_UNUSED`.

Ideally, we'd move the definition of `UNUSED` into "git-compat-util.h"
so that it becomes in accessible to the reftable library. But this is
unfortunately not easily possible as "compat/mingw-posix.h" requires
this macro, and this header is included by "compat/posix.h".

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