Remove information on grading from the documentation and code.

The feature doesn't work at the moment, and nobody seems to be relying on it.

README.md

@@ -213,10 +213,6 @@ or their score.
 Things that could make a poll's previous answers ambiguous include adding or removing a question, or adding or
 removing an answer.
 
-## Grading
-
-Each block has a score value of 1, credited to the student upon completion of the block.
-
 ## Analytics
 
 Two events are fired by the XBlocks-- one for viewing the results of a poll, and one for submitting the user's choice.

poll/poll.py

@@ -97,11 +97,6 @@ class PollBase(XBlock, ResourceMixin, PublishEventMixin):
     def send_vote_event(self, choice_data):
         # Let the LMS know the user has answered the poll.
         self.runtime.publish(self, 'progress', {})
-        self.runtime.publish(self, 'grade', {
-            'value': 1,
-            'max_value': 1,
-            }
-        )
         # The SDK doesn't set url_name.
         event_dict = {'url_name': getattr(self, 'url_name', '')}
         event_dict.update(choice_data)