[search][models] Fix warning from Django

limit_choices_to has no effect on ManyToManyField with a
through model.

beat/web/search/models.py

@@ -132,7 +132,6 @@ class Leaderboard(models.Model):
     notify = models.ManyToManyField(User, limit_choices_to={'is_active': True}, blank=True, help_text='If set, and the leader board changes, an e-mail notification will be sent to people on this list, every time it changes.')
 
     experiments = models.ManyToManyField(Experiment,
-                                         limit_choices_to={'status': Experiment.DONE},
                                          through='Rank',
                                          related_name='leaderboards',
                                          blank=True,