Remove ambiguous WeakPtr initialization from Graphics class. Closes #1459.

Source/Urho3D/Graphics/Direct3D11/D3D11Graphics.cpp

@@ -199,7 +199,6 @@ Graphics::Graphics(Context* context) :
     Object(context),
     impl_(new GraphicsImpl()),
     window_(0),
-    windowIcon_(0),
     externalWindow_(0),
     width_(0),
     height_(0),

Source/Urho3D/Graphics/Direct3D9/D3D9Graphics.cpp

@@ -243,7 +243,6 @@ Graphics::Graphics(Context* context) :
     Object(context),
     impl_(new GraphicsImpl()),
     window_(0),
-    windowIcon_(0),
     externalWindow_(0),
     width_(0),
     height_(0),

Source/Urho3D/Graphics/OpenGL/OGLGraphics.cpp

@@ -223,7 +223,6 @@ Graphics::Graphics(Context* context_) :
     Object(context_),
     impl_(new GraphicsImpl()),
     window_(0),
-    windowIcon_(0),
     externalWindow_(0),
     width_(0),
     height_(0),