Since https://github.com/OpenRailAssociation/netzgrafik-editor-frontend/commit/308028674b747f849c418ca6c316fa6af83836d1, we no longer need to handle target → source trainrun sections. Still, a lot of the codebase has special cases for these. Audit the codebase to find spots with outdated logic and simplify these. Example change: https://github.com/OpenRailAssociation/netzgrafik-editor-frontend/pull/690
