Remove wait_for_completion parameter from Renderer::SaveScreenshot

This is now unused. Seems like it was an improper fix
(there would be a race if saving the screenshot took longer
than 2 seconds) back when it was used too.

Source/Core/Core/Core.h

@@ -112,8 +112,8 @@ void SetState(State state);
 State GetState();
 void WaitUntilDoneBooting();
 
-void SaveScreenShot(bool wait_for_completion = false);
-void SaveScreenShot(std::string_view name, bool wait_for_completion = false);
+void SaveScreenShot();
+void SaveScreenShot(std::string_view name);
 
 void Callback_WiimoteInterruptChannel(int number, u16 channel_id, const u8* data, u32 size);