workspacesView: Make sure to destroy non-existing workspaces

Even if they're in the end of the list. So far we've managed to not be affected
by this bug because until GNOME 3.38, workspaces didn't have a background, and
there was no way to navigate to these leftover workspaces, but with the proposed
overview changes for GNOME 40 it'll be very much visible.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1560>

js/ui/workspacesView.js

@@ -222,6 +222,11 @@ class WorkspacesView extends WorkspacesViewBase {
             }
         }
 
+        for (let j = this._workspaces.length - 1; j >= newNumWorkspaces; j--) {
+            this._workspaces[j].destroy();
+            this._workspaces.splice(j, 1);
+        }
+
         this._updateScrollPosition();
     }