xfs: remove the m_readio_* fields in struct xfs_mount

m_readio_blocks is entirely unused, and m_readio_blocks is only used in
xfs_stat_blksize in a max statements that is a no-op as it always has
the same value as m_writeio_log.

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>