This PR does the following: 1. Suppresses underground buildings from rendering, when such buildings are tagged `location=underground`. Based on the wiki documentation, the `layer` tag does not determine whether a feature is above of below ground. 2. Unifies the building and building relation imposm exclusion mappings. The following underground building feature in Luxembourg is used as the test object: https://www.openstreetmap.org/way/582873794
building
Docs
Read the layer documentation at http://openmaptiles.org/schema#building

