Remove class-factory from wsgischema.xml for Zope > 6.0 support

This was also removed from the original wsgischema from Zope in
zopefoundation/Zope#1290

Fixes plone#212

Author: perrinjerome

news/212.bugfix

@@ -0,0 +1,4 @@
+Remove class-factory from wsgischema.xml for Zope > 6.0 support.
+
+See `issue 212 <https://github.com/plone/plone.recipe.zope2instance/issues/212>`_.
+[perrinjerome]

src/plone/recipe/zope2instance/wsgischema.xml

@@ -76,17 +76,6 @@
       </description>
     </key>
 
-    <key default="Zope2.Startup.datatypes.simpleClassFactory"
-         name="class-factory"
-         datatype=".importable_name"
-    >
-      <description>
-       Change the class factory function a database uses on a
-       per-database basis to support different class factory policy.
-       Use a Python dotted-path name to specify the class factory function.
-      </description>
-    </key>
-
     <key name="container-class"
          datatype="string"
     >