changelog: remove copy of revlog.nodemap()
It's been there since 2012, "clfilter: introduce `filteredrevs` attribute
on changelog." I don't think we can apply changelog filtering to nodemap at
this level, so this patch removes the nodemap stub completely.
Issue351: mq: qrefresh can create extra revlog entry
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ hg qinit
$ echo b > b
$ hg ci -A -m foo
adding b
$ echo cc > b
$ hg qnew -f foo.diff
$ echo b > b
$ hg qrefresh
$ hg debugindex b
rev linkrev nodeid p1 p2
0 0 1e88685f5dde 000000000000 000000000000