Bug 1596391 - Remove a dead style system function. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D52990

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

layout/style/GeckoBindings.cpp

@@ -1577,10 +1577,6 @@ FontSizePrefs Gecko_GetBaseSize(nsAtom* aLanguage) {
   return sizes;
 }
 
-const Element* Gecko_GetBindingParent(const Element* aElement) {
-  return aElement->GetBindingParent();
-}
-
 static StaticRefPtr<UACacheReporter> gUACacheReporter;
 
 namespace mozilla {

layout/style/GeckoBindings.h

@@ -561,10 +561,6 @@ mozilla::StyleGenericFontFamily Gecko_nsStyleFont_ComputeDefaultFontType(
 
 mozilla::FontSizePrefs Gecko_GetBaseSize(nsAtom* lang);
 
-// XBL related functions.
-const mozilla::dom::Element* Gecko_GetBindingParent(
-    const mozilla::dom::Element*);
-
 struct GeckoFontMetrics {
   nscoord mChSize;  // -1.0 indicates not found
   nscoord mXSize;