xfs: remove the now unused ->data_entry_p method

Now that all users use the data_entry_offset field this method is
unused and can be removed.

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>