[debugger] remove mirror cache and v8::Debug::GetMirror.

src/debug/debug.h

Index: src/debug/debug.h
diff --git a/src/debug/debug.h b/src/debug/debug.h
index 7af4502f8bb9858203ae8195bd093fe3c0838912..802e630e133b59adad4809ee9dc97fecfa4462c2 100644
--- a/src/debug/debug.h
+++ b/src/debug/debug.h
@@ -501,9 +501,6 @@ class Debug {
   MUST_USE_RESULT MaybeHandle<Object> MakeAsyncTaskEvent(Handle<Smi> type,
                                                          Handle<Smi> id);
 
-  // Mirror cache handling.
-  void ClearMirrorCache();
-
   void ProcessCompileEvent(v8::DebugEvent event, Handle<Script> script);
   void ProcessDebugEvent(v8::DebugEvent event, Handle<JSObject> event_data);