Renderer: Merge screenshot logic into VideoCommon.

Source/Core/VideoBackends/Software/SWRenderer.h

@@ -40,8 +40,4 @@ public:
                    u32 color, u32 z) override;
 
   void ReinterpretPixelData(unsigned int convtype) override {}
-  bool SaveScreenshot(const std::string& filename, const TargetRectangle& rc) override
-  {
-    return true;
-  };
 };