fix(systemtrayicon): don't set an invalid and useless icon on GTK

* The initial status icon to set on GTK didn't exist (anymore?).
* GTK resources aren't compatible with Qt's, so the resource lookup
  couldn't work anyway, even if it did exist.
* The caller calls SystemTryIcon::setIcon() right after instancing it
  anyway, so there's no need for an initial icon.

This fixes a runtime critical warning from GTK as we tried to unref a
NULL icon, which is invalid.

Fixes #3154.

src/widget/systemtrayicon.cpp

@@ -66,10 +66,7 @@ SystemTrayIcon::SystemTrayIcon()
         backendType = SystrayBackendType::GTK;
         gtk_init(nullptr, nullptr);
 
-        // No ':' needed in resource path!
-        GdkPixbuf *pixbuf = gdk_pixbuf_new_from_resource("/img/icon.png", NULL);
-        gtkIcon = gtk_status_icon_new_from_pixbuf(pixbuf);
-        g_object_unref(pixbuf);
+        gtkIcon = gtk_status_icon_new();
 
         gtkMenu = gtk_menu_new();