xfs: remove unused transaction callback variables

We don't do callbacks at transaction commit time, no do we have any
infrastructure to set up or run such callbacks, so remove the
variables and typedefs for these operations. If we ever need to add
callbacks, we can reintroduce the variables at that time.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>