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.
Merge pull request #157 from edx/feature/cale/remove-stores-state

Remove the stores_state field from XModule