Remove highway=runway and highway=taxiway rendering

[pnorman]

These tags are wrong and there are no occurances in the database.

See #204

[tyrasd]

are you sure this isn't actually used to render the refs of runways and taxiways? I see that highway and aeroway are coalesced in the sql:

coalesce(highway,aeroway) as highway

In that case: wouldn't it make sense to use the "standard" feature=key_value coalescing scheme here?

[pnorman]

hmm, good point. I checked the rendering at the local airport but I'm not sure if I checked the refs

[gravitystorm]

@tyrasd yes, this is just an artifact of the coalesce, which should be changed to follow the feature=key_value convention.