clean: delete obsolete unlinking of .hg/graftstate
The responsibility for clearing it is now in
`cmdutil.clearunfinished()`, so we shouldn't have to unlink it in
`hg.clean()`.
Differential Revision: https://phab.mercurial-scm.org/D7992
#require docutils gettext $ $TESTDIR/check-gendoc it checking for parse errors