graft: use cmdstate.delete() to delete the state file

Previous patches start using state.cmdstate() class for statefiles. The class
has a function delete() to delete the state file. This patch replaces the
existing repo.vfs.unlink() with that delete() function.

Differential Revision: https://phab.mercurial-scm.org/D3656

mercurial/commands.py

@@ -2379,7 +2379,7 b' def _dograft(ui, repo, *revs, **opts):'
 
     # remove state when we complete successfully
     if not opts.get('dry_run'):
-        repo.vfs.unlinkpath('graftstate', ignoremissing=True)
+        graftstate.delete()
 
     return 0