Remove unused aeroway=airport

aeroway=airport has [0 uses](http://taginfo.openstreetmap.org/tags/aeroway=airport) and for years has been [described on the wiki](https://wiki.openstreetmap.org/wiki/Tag%3Aaeroway%3Dairport) as specific to the Mapnik stylesheet.

[aeroway=aerodrome](http://taginfo.openstreetmap.org/tags/aeroway=aerodrome) is the tag that's in use. It's rendered with text, a symbol, and a landcover.