direct-io: remove IOCB_NOWAIT support

direct-io: remove IOCB_NOWAIT support

None of the file systems using the legacy direct I/O code actually sets
FMODE_NOWAIT, and if they did this would not work, as the write locking
could not handle the retry.  Remove this dead code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://patch.msgid.link/20260518063336.507369-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>