Optimize VesselModules

[siimav]

The two FAR VesselModules should only be active on loaded vessels. In particular FlightGUI.FixedUpdate() was rather expensive with many vessels in flight. In my test save this reduced per frame call count down from 99 to 2; median frametime from 0.83 ms to 0.23ms. Additionally FlightGUI defined its own _vessel field instead of using vessel so I took the liberty of removing that.