windowPreview: Remove slotId property

We're no longer using that property, so clean up a bit and remove the
last occurences of it.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1351

js/ui/windowPreview.js

@@ -229,7 +229,6 @@ var WindowPreview = GObject.registerClass({
 
         this._delegate = this;
 
-        this.slotId = 0;
         this._stackAbove = null;
 
         this._windowContainer.layout_manager.connect(

js/ui/workspace.js

@@ -601,8 +601,6 @@ var WorkspaceLayout = GObject.registerClass({
             const [x, y, width, height] = this._windowSlots[index];
             const windowInfo = this._windows.get(child);
 
-            child.slotId = index;
-
             if (windowInfo.metaWindow.showing_on_its_workspace()) {
                 workspaceBox.x1 = child.boundingBox.x - this._workarea.x;
                 workspaceBox.x2 = workspaceBox.x1 + child.boundingBox.width;