Delete pending scheduled messages when leaving a group.

app/src/main/java/org/thoughtcrime/securesms/groups/GroupManager.java

@@ -134,6 +134,8 @@ public final class GroupManager {
         throw new GroupChangeFailedException();
       }
     }
+
+    SignalDatabase.recipients().getByGroupId(groupId).ifPresent(id -> SignalDatabase.messages().deleteScheduledMessages(id));
   }
 
   @WorkerThread