Remove external room window feature

At its best, this worked ok-ish, though it was always missing basic features.

It's also a massive memory leak and significantly complicates the codebase.
(Which is not yet cleaned up by this commit)

Currently, it is entirely broken and noone noticed or cared enough to report or fix that.

BUG: 455984

src/roommanager.cpp

@@ -250,12 +250,6 @@ void RoomManager::openRoomForActiveConnection()
     }
 }
 
-void RoomManager::openWindow(NeoChatRoom *room)
-{
-    // forward the call to QML
-    Q_EMIT openRoomInNewWindow(room);
-}
-
 UriResolveResult RoomManager::visitUser(User *user, const QString &action)
 {
     if (action == "mention"_ls || action.isEmpty()) {