Remove more dead code after Object.observe removal

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index 2eb5bcae55a25ed8c9580c14ec9e41907c8e8317..d0b8c9ef4193895d994ad4125e64dd33249327c7 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -1898,7 +1898,7 @@ void Debug::CallEventCallback(v8::DebugEvent event,
                               exec_state,
                               event_data,
                               event_listener_data_ };
-    Handle<JSReceiver> global(isolate_->global_proxy());
+    Handle<JSReceiver> global = isolate_->global_proxy();
     Execution::TryCall(isolate_, Handle<JSFunction>::cast(event_listener_),
                        global, arraysize(argv), argv);
   }