ublk: remove check IO_URING_F_SQE128 in ublk_ch_uring_cmd

sizeof(struct ublksrv_io_cmd) is 16bytes, which can be held in 64byte SQE,
so not necessary to check IO_URING_F_SQE128.

With this change, we get chance to save half SQ ring memory.

Fixed: 71f28f31

 ("ublk_drv: add io_uring based userspace block driver")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230220041413.1524335-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>