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 41abfe1 (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 bfc2f9e (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>

Author: peff

builtin/gc.c

@@ -260,7 +260,7 @@ static int pack_refs_condition(UNUSED struct gc_config *cfg)
 	return 1;
 }
 
-static int maintenance_task_pack_refs(MAYBE_UNUSED struct maintenance_run_opts *opts,
+static int maintenance_task_pack_refs(struct maintenance_run_opts *opts,
 				      UNUSED struct gc_config *cfg)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;