Only use admin_levels from ways.

Effectively a rebase of #348.

This isn't ideal, since we should be able to (somehow) figure out
the max(admin_level) from all the ways and relations automatically,
but the osm2pgsql database layout doesn't make it easy.
Since mappers are commonly doing this algorithm anyway by hand
 - well, hey-ho. It's still an improvement in the end result.