VideoCommon: Make dst_alpha state implicit.

Source/Core/VideoBackends/D3D12/VertexManager.h

@@ -29,7 +29,7 @@ protected:
 private:
   void PrepareDrawBuffers(u32 stride);
   void Draw(u32 stride);
-  void vFlush(bool use_dst_alpha) override;
+  void vFlush() override;
 
   u32 m_vertex_draw_offset;
   u32 m_index_draw_offset;