drop KX11Extras::compositingActive(), fixes #344
Without this MR, log gets filled with kf.windowsystem: static bool KX11Extras::compositingActive() may only be used on X11
lines. I've finally found the time to test and confirm that this MR removes those un-needed logs.
@devinlin @nicolasfella IMO, those logs lines do not harm, but are boring while debugging other issues, so I'd rather merge and make these logs lines disappear... I let the decision to your experience