Add removing of forgotten rooms

lib/src/Room.dart

@@ -233,6 +233,7 @@ class Room {
 
   /// Call the Matrix API to forget this room if you already left it.
   Future<dynamic> forget() async {
+    client.store.forgetRoom(id);
     dynamic res = await client.connection
         .jsonRequest(type: "POST", action: "/client/r0/rooms/${id}/forget");
     if (res is ErrorResponse) client.connection.onError.add(res);

lib/src/Store.dart

@@ -576,6 +576,11 @@ class Store {
     return directChats;
   }
 
+  Future<void> forgetRoom(String roomID) async {
+    await db.rawDelete("DELETE FROM Rooms WHERE id=?", [roomID]);
+    return;
+  }
+
   /// The database sheme for the Client class.
   static final String ClientsScheme = 'CREATE TABLE IF NOT EXISTS Clients(' +
       'client TEXT PRIMARY KEY, ' +