xfs: remove an outdated comment for xfs_inode_item_committing

The function now does something, and that something is central to our
inode logging scheme.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>