io_uring: Remove unused function req_ref_put

[ upstream commmit c84b8a3f ]

Fix the following clang warnings:

fs/io_uring.c:1195:20: warning: unused function 'req_ref_put'
[-Wunused-function].

Fixes: aa43477b

 ("io_uring: poll rework")
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220113162005.3011-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[pavel: backport]
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>