remove GTFS-Flex v1 fields #7
Description
Currently, https://gtfs.mfdz.de/hbg3.merged.gtfs.zip contains at least one GTFS-Flex v1 field, drt_advance_book_min:
head -n 1 trips.txt
# "route_id","trip_id","service_id","trip_headsign","direction_id","block_id","shape_id","drt_advance_book_min","bikes_allowed","peak_offpeak"