xfs: kill unsupported superblock versions

We don't support filesystems older than dir v2 support, so we
always know about v2 inodes and nlink and other feature bits.
Strip out all the old, unnecessary feature support stuff from
xfs_repair in preparation for merging the 3.16 libxfs code from
the kernel.

Signed-off-by: Dave Chinner <dchinner@redhat.com>