Calendar: Meeting deletion can sometimes send duplicate cancel messages

Servers with "save-schedule" capability send meeting change notifications
on their own. When deleting a meeting and sending specific cancellation
mail, it doesn't make much sense to let the server send another cancellation
mail by itself, due to the deletion, thus let the calendar backend know
that it can avoid sending the message.

src/calendar/gui/e-calendar-view.c

@@ -202,6 +202,9 @@ calendar_view_delete_event (ECalendarView *cal_view,
 				itip_send_component_with_model (model, I_CAL_METHOD_CANCEL,
 					comp, client, NULL, NULL,
 					NULL, E_ITIP_SEND_COMPONENT_FLAG_STRIP_ALARMS);
+
+				/* attendees will know by the above cancel message */
+				op_flags = E_CAL_OPERATION_FLAG_DISABLE_ITIP_MESSAGE;
 			} else {
 				op_flags = E_CAL_OPERATION_FLAG_DISABLE_ITIP_MESSAGE;
 			}