iomap: Remove IOMAP_DIO_NOSYNC unused dio flag

IOMAP_DIO_NOSYNC earlier was added for use in btrfs. But it seems for
aio dsync writes this is not useful anyway. For aio dsync case, we
we queue the request and return -EIOCBQUEUED. Now, since IOMAP_DIO_NOSYNC
doesn't let iomap_dio_complete() to call generic_write_sync(),
hence we may lose the sync write.

Hence kill this flag as it is not in use by any FS now.

Tested-by: Disha Goel <disgoel@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>