io-wq: remove unused io_wqe_need_worker() function

A previous commit removed the need for this, but overlooked that we no
longer use it at all. Get rid of it.

Fixes: 685fe7fe ("io-wq: eliminate the need for a manager thread")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>