workqueue: Fold rebind_worker() within rebind_workers()

!CONFIG_SMP builds complain about rebind_worker() being unused. Its only
user, rebind_workers() is indeed only defined for CONFIG_SMP, so just fold
the two lines back up there.

Link: http://lore.kernel.org/r/20230113143102.2e94d74f@canb.auug.org.au
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>