Remove the barely used macro assemblers after merging their contents to the base

runtime/vm/assembler_macros_mips.cc

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-#include "vm/globals.h"
-#if defined(TARGET_ARCH_MIPS)
-
-#include "vm/assembler_macros.h"
-
-namespace dart {
-
-void AssemblerMacros::TryAllocate(Assembler* assembler,
-                                  const Class& cls,
-                                  Label* failure,
-                                  bool near_jump,
-                                  Register instance_reg) {
-  UNIMPLEMENTED();
-}
-
-
-void AssemblerMacros::EnterDartFrame(Assembler* assembler,
-                                     intptr_t frame_size) {
-  UNIMPLEMENTED();
-}
-
-
-void AssemblerMacros::EnterStubFrame(Assembler* assembler) {
-  UNIMPLEMENTED();
-}
-
-}  // namespace dart
-
-#endif  // defined TARGET_ARCH_MIPS
-