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

src/debug/debug.js

Index: src/debug/debug.js
diff --git a/src/debug/debug.js b/src/debug/debug.js
index 88579c2108df7923f26052e403b986f3456bb6c3..6993274f09efe2bf1bfcbb76b393f9bc0025e80b 100644
--- a/src/debug/debug.js
+++ b/src/debug/debug.js
@@ -12,18 +12,11 @@ var FrameMirror = global.FrameMirror;
 var GlobalArray = global.Array;
 var GlobalRegExp = global.RegExp;
 var IsNaN = global.isNaN;
-var LookupMirror = global.LookupMirror;
 var MakeMirror = global.MakeMirror;
 var MathMin = global.Math.min;
 var Mirror = global.Mirror;
-var MirrorType;
-var ParseInt = global.parseInt;
 var ValueMirror = global.ValueMirror;
 
-utils.Import(function(from) {
-  MirrorType = from.MirrorType;
-});
-
 //----------------------------------------------------------------------------
 
 // Default number of frames to include in the response to backtrace request.