Revert "io_uring: disallow self-propelled ring polling"

This reverts commit 7fdbc5f0.

This patch dealt with a subset of the real problem, which is a potential
circular dependency on the wakup path for io_uring itself. Outside of
io_uring, eventfd can also trigger this (see details in 03e02acd)
and so can epoll (see details in caf1aeaf

). Now that we have a
generic solution to this problem, get rid of the io_uring specific
work-around.

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