Remove the "Conditionally Forbidden" presence condition from the spec

[isabelle-dr]

Context

When a field is described as Conditionally Required in the spec, three cases exist:

1. A field or file is required under a certain condition, optional otherwise. It is the case for the calendar.txt, calendar_dates.txt, and levels.txt files, for the fields stops.stop_name, stops.stop_lat, stops.stop_lon, stops.zone_id and for many other fields.

2. A field or file is required under a certain condition, forbidden otherwise. it is the case for fare_transfer_rules.duration_limit_type, translations.record_id, and translations.field_value.

3. A field or file is required under a certain condition, forbidden under another condition, and optional otherwise. It is the case for stops.parent_station.

Problem

In the Fares v2 base implementation PR, a new presence condition was added to the spec: Conditionally Forbidden, and was applied to fare_transfer_rules.transfer_count. This field has the behavior described in category 2 above: required under a certain condition, forbidden otherwise.
We believe this field was initially designed as: forbidden under a certain condition, optional otherwise, and then changed when modifications were made to the PR.
This adds unnecessary complexity to the spec, this condition should only be used if a field is either forbidden or optional.

Solution

Remove the Conditionally Forbidden presence condition from the spec and describe fare_transfer_rules.transfer_count as Conditionally Required.

I'm happy to open a PR to do this fix if there is agreement from the community.

[emmambd]

👋 I'm going through our issue backlog to see if there's anything relevant to roll into the scope for #376. It looks like #376 may be a good opportunity to remove Conditionally Forbidden as well since there was 4 thumbs up in favor of this issue and no dissent.

Checking in with @isabelle-dr and those who +1'd: @westontrillium, @botanize, @timMillet, @jfabi

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It may be closed manually after one month of inactivity. Thank you for your contributions.

[isabelle-dr]

We can close - we can now make use of Conditionally Forbidden