[turbofan] Factor C call descriptor building into compiler/c-linkage.cc.

src/compiler/linkage.cc

 // Copyright 2014 the V8 project 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 "src/code-stubs.h"
 #include "src/compiler.h"
 #include "src/compiler/common-operator.h"
 #include "src/compiler/linkage.h"
 #include "src/compiler/node.h"
 #include "src/compiler/pipeline.h"
@@ skipping 262 matching lines @@
 
 CallDescriptor* Linkage::GetStubCallDescriptor(
     Isolate* isolate, Zone* zone, const CallInterfaceDescriptor& descriptor,
     int stack_parameter_count, CallDescriptor::Flags flags,
     Operator::Properties properties, MachineType return_type) {
   UNIMPLEMENTED();
   return NULL;
 }
 
 
-CallDescriptor* Linkage::GetSimplifiedCDescriptor(Zone* zone,
-                                                  const MachineSignature* sig) {
-  UNIMPLEMENTED();
-  return NULL;
-}
 #endif  // !V8_TURBOFAN_BACKEND
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8