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/qml/ContextMenu.qml

@@ -27,18 +27,6 @@ Loader {
     Component {
         id: regularMenu
         QQC2.Menu {
-            QQC2.MenuItem {
-                id: newWindow
-                text: i18n("Open in New Window")
-                icon.name: "window-new"
-                onTriggered: RoomManager.openWindow(room)
-                visible: !Kirigami.Settings.isMobile
-            }
-
-            QQC2.MenuSeparator {
-                visible: newWindow.visible
-            }
-
             QQC2.MenuItem {
                 text: room.isFavourite ? i18n("Remove from Favorites") : i18n("Add to Favorites")
                 icon.name: room.isFavourite ? "bookmark-remove" : "bookmark-new"