remove leftovers of shared read-only support

We never supported shared read-only filesystems, so remove the dead
code left over from IRIX for it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>