io_uring: kill unused IO_IOPOLL_BATCH

IO_IOPOLL_BATCH is not used, delete it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b2bdf19dbee2c9fc8865bbab9412135a14e24a64.1628471125.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>