Remove WindowTreeClientDelegate::GetCaptureClient() and WindowTreeClient::GetCaptureClient().

ash/mus/window_manager_unittest.cc

Index: ash/mus/window_manager_unittest.cc
diff --git a/ash/mus/window_manager_unittest.cc b/ash/mus/window_manager_unittest.cc
index 168bb4b702fb215d3163cd0f267df51e08f28a84..4c2c0cd0e88246778ebfd775b94812bdbd1a1108 100644
--- a/ash/mus/window_manager_unittest.cc
+++ b/ash/mus/window_manager_unittest.cc
@@ -49,9 +49,6 @@ class WindowTreeClientDelegate : public aura::WindowTreeClientDelegate {
   void OnLostConnection(aura::WindowTreeClient* client) override {}
   void OnPointerEventObserved(const ui::PointerEvent& event,
                               aura::Window* target) override {}
-  aura::client::CaptureClient* GetCaptureClient() override {
-    return wm_state_.capture_controller();
-  }
   aura::PropertyConverter* GetPropertyConverter() override {
     return &property_converter_;
   }