Vulkan: Faster path for decoding XFB data

Using a texel buffer as the copy destination removes the need to copy to
an intermediate texture first.

Source/Core/VideoBackends/Vulkan/TextureCache.h

@@ -48,7 +48,6 @@ public:
 
   static TextureCache* GetInstance();
 
-  StreamBuffer* GetUploadBuffer() const { return m_texture_upload_buffer.get(); }
   TextureConverter* GetTextureConverter() const { return m_texture_converter.get(); }
   bool Initialize();