Remove unused files (#58)

* Remove unused files

This change removes the following files/folders:

- About.md (old gtfs.org)
- Background.md (old gtfs.org)
- Schedule folder (old gtfs.org)
- Realtime folder (old gtfs.org)
- Resource library folder

* Restores gtfs-realtime.proto

* Revert last commit

gtfs-realtime.proto will be added in a separate PR