dagutil: remove nodeset()
AFAICT this was unused since the code was introduced by
cb98fed52495
in 2011!
Differential Revision: https://phab.mercurial-scm.org/D4309
$ 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