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

mercurial/hg.py

@@ -1041,7 +1041,6 b' def clean(repo, node, show_stats=True, q'
     """forcibly switch the working directory to node, clobbering changes"""
     stats = updaterepo(repo, node, True)
     assert stats.unresolvedcount == 0
-    repo.vfs.unlinkpath(b'graftstate', ignoremissing=True)
     if show_stats:
         _showstats(repo, stats, quietempty)