pt_diagrams: ignore nodes with public_transport=platform

The new public transport schema often maps stops using a pair of nodes: a public_transport=stop_position node on the road/rails and a public_transport=platform on the pavement or traffic island.

This change makes pt_diagrams ignore nodes with public_transport=platform (even if they contain another tag that would declare them a stop), since there is very probably a node with public_transport=stop_position nearby, and taking both into account creates duplicate stops.