VideoCommon: Drop SetDitherMode()

It was a no-op on all backends apart from GL anyhow.

Source/Core/VideoBackends/Vulkan/Renderer.cpp

@@ -1383,10 +1383,6 @@ void Renderer::ResetSamplerStates()
   g_object_cache->ClearSamplerCache();
 }
 
-void Renderer::SetDitherMode()
-{
-}
-
 void Renderer::SetInterlacingMode()
 {
 }