network: Don't destroy wireless dialog twice

The dialog doesn't change the `destroyOnClose` property from its default,
so it is already destroyed automatically on close. So if we also destroy
it explicitly, we end up (rightfully) with one of gjs' infamous "invalid
access" warnings.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/430

js/ui/status/network.js

@@ -1236,7 +1236,6 @@ var NMDeviceWireless = class {
     }
 
     _dialogClosed() {
-        this._dialog.destroy();
         this._dialog = null;
     }