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.h

@@ -148,13 +148,6 @@ public:
      */
     Q_INVOKABLE void loadInitialRoom();
 
-    /**
-     * @brief Open a new window with the given room.
-     *
-     * The open window will have its own message list for the given room.
-     */
-    Q_INVOKABLE void openWindow(NeoChatRoom *room);
-
     /**
      * @brief Leave the room and close it if it is open.
      */