refactor(pipeline): remove old C# generator classes after Python pipeline validates

[nigel-hall-codes]

Delete OsmParser.cs, ElevationParser.cs, GeoProjection.cs, RoadMeshGenerator.cs, IntersectionMeshGenerator.cs, SidewalkMeshGenerator.cs, TerrainGenerator.cs, BuildingGenerator.cs. Trim PipelineTypes.cs to types still used by the importer.

Design: sdlc/#61/design.md — C# Changes / What gets deleted.

Discovered from #61.