gc: drop MAYBE_UNUSED annotation from used parameter

gc: drop MAYBE_UNUSED annotation from used parameter

The "opts" parameter is always used, so marking it with MAYBE_UNUSED is
just confusing.

This annotation goes back to 41abfe15d9 (maintenance: add pack-refs
task, 2021-02-09), when it really was unused. Back then we did not have
the UNUSED macro that would complain if the code changed to use the
parameter. So when we started using it in bfc2f9eb8e (builtin/gc:
forward git-gc(1)'s `--auto` flag when packing refs, 2024-03-25), nobody
noticed.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>