blink_gc_plugin: retire overloaded traceImpl detection and handling.

tools/clang/blink_gc_plugin/tests/traceimpl_error.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_error.h"
 
 namespace blink {
 
 void TraceImplExternWithUntracedMember::Trace(Visitor* visitor) {
-  TraceImpl(visitor);
-}
-
-template <typename VisitorDispatcher>
-inline void TraceImplExternWithUntracedMember::TraceImpl(
-    VisitorDispatcher visitor) {
   // Should get a warning as well.
 }
 
 void TraceImplExternWithUntracedBase::Trace(Visitor* visitor) {
-  TraceImpl(visitor);
-}
-
-template <typename VisitorDispatcher>
-inline void TraceImplExternWithUntracedBase::TraceImpl(
-    VisitorDispatcher visitor) {
   // Ditto.
 }
 
 }