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 black
$ cd $RUNTESTDIR/..
$ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`