Revert "io_uring: add buffer selection support to IORING_OP_NOP"

This reverts commit 3d200242.

Buffer selection with nops was used for debugging and benchmarking but
is useless in real life. Let's revert it before it's released.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c5012098ca6b51dfbdcb190f8c4e3c0bf1c965dc.1655224415.git.asml.silence@gmail.com


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