Removes some dead code

ash/host/ash_window_tree_host.h

Index: ash/host/ash_window_tree_host.h
diff --git a/ash/host/ash_window_tree_host.h b/ash/host/ash_window_tree_host.h
index 371d100646f1230e27b58c08ccd99ba135698aaa..2459aea36d6a5e46c70af7da6f2bb8d9bb21887c 100644
--- a/ash/host/ash_window_tree_host.h
+++ b/ash/host/ash_window_tree_host.h
@@ -8,7 +8,6 @@
 #include <memory>
 
 #include "ash/ash_export.h"
-#include "base/callback_forward.h"
 
 namespace aura {
 class WindowTreeHost;
@@ -29,15 +28,12 @@ class RootWindowTransformer;
 
 class ASH_EXPORT AshWindowTreeHost {
  public:
-  using Factory =
-      base::Callback<AshWindowTreeHost*(const AshWindowTreeHostInitParams&)>;
   AshWindowTreeHost();
   virtual ~AshWindowTreeHost() {}
 
   // Creates a new AshWindowTreeHost. The caller owns the returned value.
   static AshWindowTreeHost* Create(
       const AshWindowTreeHostInitParams& init_params);
-  static void SetFactory(const Factory& factory);
 
   void set_input_method_handler(InputMethodEventHandler* input_method_handler) {
     input_method_handler_ = input_method_handler;