histedit: drop unnecessary check for "self.node is not None"

We are doing hex(self.node) just a few lines up, so it shouldn't be
None. The only way it could be none is if it was reassigned in
between. The only way that can happen is if the user had put a
"ffff..." wdirhex revision in the histedit script. This code is much
older than the "ffff..." identifier, so I'm confident it's not there
to handle that case. I'll let someone else add proper checks for
"ffff..." if they care enough.

Differential Revision: https://phab.mercurial-scm.org/D3157

hgext/histedit.py

@@ -448,8 +448,7 b' class histeditaction(object):'
         except error.RepoError:
             raise error.ParseError(_('unknown changeset %s listed')
                               % ha[:12])
-        if self.node is not None:
-            self._verifynodeconstraints(prev, expected, seen)
+        self._verifynodeconstraints(prev, expected, seen)
 
     def _verifynodeconstraints(self, prev, expected, seen):
         # by default command need a node in the edited list