io_uring: kill io_uring userspace examples

There are tons of io_uring tests and examples in liburing and on the
Internet. If you're looking for a benchmark, io_uring-bench.c is just an
acutely outdated version of fio/io_uring. And for basic condensed init
template for likes of selftests take a peek at io_uring_zerocopy_tx.c.

Kill tools/io_uring/, it's a burden keeping it here.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7c740701d3b475dcad8c92602a551044f72176b4.1691543666.git.asml.silence@gmail.com


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