Bug 1621258 [Wayland] Don't print 'Missing GtkWidget' warning, r=jhorak

Differential Revision: https://phabricator.services.mozilla.com/D66221

--HG--
extra : moz-landing-system : lando

widget/gtk/WidgetStyleCache.cpp

@@ -760,7 +760,6 @@ GtkWidget* GetWidget(WidgetNodeType aAppearance) {
     // Some widgets (MOZ_GTK_COMBOBOX_SEPARATOR for instance) may not be
     // available or implemented.
     if (!widget) {
-      NS_WARNING(nsPrintfCString("Missing GtkWidget %d\n", aAppearance).get());
       return nullptr;
     }
     // In GTK versions prior to 3.18, automatic invalidation of style contexts