dagutil: remove ability to invert instances
The previous commit removed the last consumer of this feature.
.. api:: remove inverse() methods from classes in dagutil
Differential Revision: https://phab.mercurial-scm.org/D4323
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state