Revert "Add explicit v8 locker"

This reverts commit 11d1eca9f3.

It sporadically (but reproducibly) triggers an assert inside V8:

  Fatal error in /path/to/node/deps/v8/src/isolate.cc, line 1857
  CHECK(CurrentPerIsolateThreadData()->isolate_ == this) failed

Needs further investigation.

src/node.cc

@@ -2633,7 +2633,6 @@ int Start(int argc, char *argv[]) {
   argv = Init(argc, argv);
 
   v8::V8::Initialize();
-  v8::Locker locker;
   v8::HandleScope handle_scope;
 
   // Create the one and only Context.