Make XModuleDescriptor and XModule act as a single class

By transparently proxying between the XModuleDescriptor and the XModule,
and between their runtimes, we can make them act as a single class, so
that we can swap in an actual XBlock instead.

common/lib/xmodule/xmodule/seq_module.py

@@ -45,9 +45,6 @@ class SequenceModule(SequenceFields, XModule):
 
         self.rendered = False
 
-    def get_instance_state(self):
-        return json.dumps({'position': self.position})
-
     def get_html(self):
         self.render()
         return self.content