Use gtfsio package to import feeds

Now that v0.0.5 is finally on CRAN, the whole [`extract_gtfs()` procedures](https://github.com/ATFutures/gtfs-router/blob/main/R/extract-gtfs.R) now have to be ditched in favour of the new [`gtfsio` pacakge](https://github.com/r-transit/gtfsio) by @dhersz.