blink_gc_plugin: retire overloaded traceImpl detection and handling.

tools/clang/blink_gc_plugin/tests/traceimpl_overloaded.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "traceimpl_overloaded.h"
 
 namespace blink {
 
 void ExternBase::Trace(Visitor* visitor) {
-  TraceImpl(visitor);
-}
-
-void ExternBase::Trace(InlinedGlobalMarkingVisitor visitor) {
-  TraceImpl(visitor);
-}
-
-template <typename VisitorDispatcher>
-inline void ExternBase::TraceImpl(VisitorDispatcher visitor) {
   visitor->Trace(x_base_);
 }
 
 void ExternDerived::Trace(Visitor* visitor) {
-  TraceImpl(visitor);
-}
-
-void ExternDerived::Trace(InlinedGlobalMarkingVisitor visitor) {
-  TraceImpl(visitor);
-}
-
-template <typename VisitorDispatcher>
-inline void ExternDerived::TraceImpl(VisitorDispatcher visitor) {
   visitor->Trace(x_derived_);
   ExternBase::Trace(visitor);
 }
 
 }