do not render ✈ icon for aerodrome=airstrip

An ongoing [tag proposal](https://osm.wiki/Proposal:Aerodrome_Classification) will replace [`aeroway=airstrip`](https://osm.wiki/Tag:aeroway=airstrip) with `aeroway=aerodrome` + `aerodome=airstrip`. If approved, this will cause airstrips to suddenly start being rendered. However, rendering airstrips was previously declined in #3853. Therefore, I think the rendering logic needs to be updated to hide the ✈ icon if `aerodome=airstrip`. Otherwise the map will start looking like this: <img src="https://wiki.openstreetmap.org/w/images/a/ac/Airstrips_rendered_identically_to_aerodromes.png" width=500 /> with every farmer's airstrip shown from zoom level 10, as prominently as an airport. For reference, this is an airstrip: it has 0 facilities and infrastructure, it's just a straight paddock that really does not deserve a prominent icon...