Resolve "Fix timezone handling with substitutions/amend dialog"

Closes #250 (closed)

aleksis/apps/chronos/schema/__init__.py

-from datetime import timezone
-
 import graphene
 from graphene_django import DjangoObjectType
 
@@ -60,26 +58,6 @@ class LessonEventType(DjangoObjectType):
     amends = graphene.Field(lambda: LessonEventType, required=False)
 
 
-class DatetimeTimezoneMixin:
-    """Handle datetimes for mutations with CalendarEvent objects.
-
-    This is necessary because the client sends timezone information as
-    ISO string which only includes an offset (+00:00 UTC) and an
-    offset is not a valid timezone. Instead we set UTC as timezone
-    here directly.
-    """
-
-    @classmethod
-    def handle_datetime_start(cls, value, name, info) -> int:
-        value = value.replace(tzinfo=timezone.utc)
-        return value
-
-    @classmethod
-    def handle_datetime_end(cls, value, name, info) -> int:
-        value = value.replace(tzinfo=timezone.utc)
-        return value
-
-
 class AmendLessonBatchCreateMutation(BaseBatchCreateMutation):
     class Meta:
         model = LessonEvent