MAINT: Remove unused parameter

This has existed since forever (762326d) but was not used even then.
It's unclear if in the interim there used to be some subclass that
used it, but anyway nowadays there isn't, so begone!

core/src/zeit/cms/content/property.py

@@ -182,7 +182,7 @@ def __get__(self, instance, class_):
         anchor = self.path.find(tree, None)
         if anchor is not None:
             for node in anchor.getparent().iterchildren(anchor.tag):
-                result.append(self._element_factory(node, tree))
+                result.append(self._element_factory(node))
         return self.result_type(elem for elem in result if elem is not None)
 
     def __set__(self, instance, value):
@@ -194,20 +194,20 @@ def __set__(self, instance, value):
                 entry.getparent().remove(entry)
         # Add new nodes:
         value = self.sorted(value)
-        self.path.setattr(tree, [self._node_factory(entry, tree) for entry in value])
+        self.path.setattr(tree, [self._node_factory(entry) for entry in value])
 
-    def _element_factory(self, node, tree):
+    def _element_factory(self, node):
         raise NotImplementedError('Implemented in sub classes.')
 
-    def _node_factory(self, entry, tree):
+    def _node_factory(self, entry):
         raise NotImplementedError('Implemented in sub classes.')
 
 
 class SimpleMultiProperty(MultiPropertyBase):
-    def _element_factory(self, node, tree):
+    def _element_factory(self, node):
         return node.text
 
-    def _node_factory(self, entry, tree):
+    def _node_factory(self, entry):
         name = str(self.path).split('.')[-1]
         return getattr(lxml.builder.E, name)(entry)