xfs: remove unused procfs code

As a part of the work to move xfs global stats from procfs to sysfs,
this patch removes the now unused procfs code that was xfs stat specific.

Signed-off-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>