Remove legacy type property from LessonSubstitution

Advances #75

aleksis/apps/chronos/models.py

@@ -509,15 +509,6 @@ class LessonSubstitution(ExtensibleModel):
         if self.subject and self.cancelled:
             raise ValidationError(_("Lessons can only be either substituted or cancelled."))
 
-    @property
-    def type_(self):
-        if self.cancelled:
-            return "cancellation"
-        elif self.cancelled_for_teachers:
-            return "cancellation_for_teachers"
-        else:
-            return "substitution"
-
     @property
     def date(self):
         week = CalendarWeek(week=self.week)