Set override to None on init_from_structure

lms/djangoapps/grades/new/subsection_grade.py

@@ -108,6 +108,7 @@ class SubsectionGrade(SubsectionGradeBase):
             self._compute_block_score(descendant_key, course_structure, submissions_scores, csm_scores)
 
         self.all_total, self.graded_total = graders.aggregate_scores(self.problem_scores.values())
+        self.override = None
         self._log_event(log.debug, u"init_from_structure", student)
         return self