io_uring: remove dead check in io_splice()

We checked for 'force_nonblock' higher up, so it's definitely false
at this point. Kill the check, it's a remnant of when we tried to do
inline splice without always punting to async context.

Fixes: 2fb3e822 ("io_uring: punt splice async because of inode mutex")
Signed-off-by: Jens Axboe <axboe@kernel.dk>