sched/wait: Remove unused bit_wait_io_timeout

bit_wait_io_timeout has been unused since 2016's
commit 62906027 ("mm: add PageWaiters indicating tasks are waiting for a page bit")

Remove it.

Signed-off-by: "Dr. David Alan Gilbert" <linux@treblig.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Link: https://lore.kernel.org/r/20241001234016.231696-1-linux@treblig.org