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/annotatable_module.py

@@ -125,6 +125,5 @@ class AnnotatableModule(AnnotatableFields, XModule):
 
 class AnnotatableDescriptor(AnnotatableFields, RawDescriptor):
     module_class = AnnotatableModule
-    stores_state = True
     template_dir_name = "annotatable"
     mako_template = "widgets/raw-edit.html"