Delete widget in WrapperLayout.unwrap()

This was removed in f5f3bf63b5 but is still
needed so that the underlying QWebEngineView is actually deleted with Qt <
5.12. Otherwise, test_spawning_an_editor_and_closing_the_tab in
test_editor_bdd.py would fail because the element doesn't actually vanish.

qutebrowser/misc/miscwidgets.py

@@ -279,6 +279,7 @@ class WrapperLayout(QLayout):
             return
         assert self._container is not None
         self._widget.setParent(None)  # type: ignore[call-overload]
+        self._widget.deleteLater()
         self._widget = None
         self._container.setFocusProxy(None)  # type: ignore[arg-type]