mainwindow: Correctly remove cards when they go away

The reference is owned by the VBox, so we remove it there and it gets
cleaned up, like other widgets.

Fixes: https://gitlab.freedesktop.org/pulseaudio/pavucontrol/-/issues/162
Part-of: <https://gitlab.freedesktop.org/pulseaudio/pavucontrol/-/merge_requests/98>

src/mainwindow.cc

@@ -1272,7 +1272,7 @@ void MainWindow::removeCard(uint32_t index) {
     if (!cardWidgets.count(index))
         return;
 
-    delete cardWidgets[index];
+    cardsVBox->remove(*cardWidgets[index]);
     cardWidgets.erase(index);
     updateDeviceVisibility();
 }