xfs: drop XFS_BTREE_CRC_BLOCKS

All existing btree types set XFS_BTREE_CRC_BLOCKS when running against a
V5 filesystem.  All currently proposed btree types are V5 only and use
the richer XFS_BTREE_CRC_BLOCKS format.  Therefore, we can drop this
flag and change the conditional to xfs_has_crc.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>