Removes zero-length self-looped edges if encountered in `nX_simple_geoms`

Removes zero-length self-looped edges if encountered in nX_simple_geoms

This resolves a situation where a converted OSM graph with a zero length edge causes downstream errors. This arises because inferring geometry for a looped OSM edge (same start and end node) implies a zero-length geom: this results in geometry.Point instead of geometry.LineString. These zero-length edges are now removed.