Remove unnecessary "duration_td" Tummy Time method.

core/models.py

@@ -202,9 +202,6 @@ class TummyTime(models.Model):
     def __str__(self):
         return 'Tummy Time'
 
-    def duration_td(self):
-        return self.end - self.start
-
     def save(self, *args, **kwargs):
         if self.start and self.end:
             self.duration = self.end - self.start

dashboard/templatetags/cards.py

@@ -155,6 +155,5 @@ def card_tummytime_day(child, date=None):
         'count': instances.count()
     }
     for instance in instances:
-        stats['total'] += timezone.timedelta(
-            seconds=instance.duration_td().seconds)
+        stats['total'] += timezone.timedelta(seconds=instance.duration.seconds)
     return {'stats': stats, 'instances': instances, 'last': instances.first()}