Remove destructor disconnect call for onFadeComplete handler

The `MainMenuState` object owns the `mFade` member field, so `mFade` gets destructed as part of `MainMenuState`. Part of the destruction for the `mFade` field is to disconnect all `Signal` handlers. Thus the manual `disconnect` is redundant.

Author: DanRStevens

appOPHD/States/MainMenuState.cpp

@@ -37,7 +37,6 @@ MainMenuState::~MainMenuState()
 	eventHandler.keyDown().disconnect({this, &MainMenuState::onKeyDown});
 
 	NAS2D::Utility<NAS2D::Mixer>::get().stopAllAudio();
-	mFade.fadeComplete().disconnect({this, &MainMenuState::onFadeComplete});
 }