xfs: remove the special COW fork handling in xfs_bmapi_read

We don't call xfs_bmapi_read for the COW fork anymore, so remove the
special casing.

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