Airliners.net looked into this a few years ago and said this "As stated, Google has a new algorithm that removes non static objects from their maps. The primary purpose is to remove cars/pedestrians from the streets/sidewalk so you can see them easier, but planes (being non static across images) also get removed."

Oddly, the roads still seem to have cars and lorries on, but the planes have been edited out (even if the airport were empty, there'd have been lots of aircraft on the engineering base!). If you zoom in on the apron and the engineering base, you can see blurry bits where the editing has happened... Weird!

EDIT: Actually, I think it must be something more deliberate than that "algorithm" explanation above. Because it doesn't happen at most airports... BHX, MAN, AMS, FRA, CWL and even JFK all still have planes in them, but Heathrow, Gatwick and Northolt don't....