Remove the stores_state field from XModule

stores_state is now only used in a single test, in conjunction with
has_score. In practice, it's never the case that stores_state is false
when has_score is true, so we can delete stores_state entirely, and just
use has_score for the grading test.

common/lib/xmodule/xmodule/randomize_module.py

@@ -94,7 +94,6 @@ class RandomizeDescriptor(RandomizeFields, SequenceDescriptor):
 
     filename_extension = "xml"
 
-    stores_state = True
 
     def definition_to_xml(self, resource_fs):