Be less aggressive when validating schedule edit form POST

Somehow deletions were requested for shifts which didn't exist...not
sure how that happened but let's just ignore instead of raise error

tailbone/views/shifts/schedule.py

@@ -83,10 +83,10 @@ class ScheduleView(TimeSheetView):
             # apply delete operations
             deleted = []
             for uuid, value in data['delete'].iteritems():
-                assert value == 'delete'
-                shift = Session.query(model.ScheduledShift).get(uuid)
-                assert shift
-                Session.delete(shift)
+                if value == 'delete':
+                    shift = self.Session.query(model.ScheduledShift).get(uuid)
+                    if shift:
+                        self.Session.delete(shift)
                         deleted.append(uuid)
 
             # apply create / update operations