blink_gc_plugin: retire overloaded traceImpl detection and handling.

tools/clang/blink_gc_plugin/tests/traceimpl.cpp

 // Copyright 2014 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.h"
 
 namespace blink {
 
 void TraceImplExtern::Trace(Visitor* visitor) {
-  TraceImpl(visitor);
-}
-
-template <typename VisitorDispatcher>
-inline void TraceImplExtern::TraceImpl(VisitorDispatcher visitor) {
   visitor->Trace(x_);
 }
 
 void TraceImplBaseExtern::Trace(Visitor* visitor) {
-  TraceImpl(visitor);
-}
-
-template <typename VisitorDispatcher>
-inline void TraceImplBaseExtern::TraceImpl(VisitorDispatcher visitor) {
   visitor->Trace(x_);
   Base::Trace(visitor);
 }
 
 }