Fix #10574 -- revert the change and don't list unavailable road types to game script.

[PeterN]

Motivation / Problem

See #10574.

Description

Re-allow town-buildable to override hidden, but do check for availability for game script listing.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[PeterN]

Updated to remove anything to do with rail types.

I don't know if this change is the way forward, it's just a potential solution.

[Andrew350]

FWIW, as a NewGRF developer affected by the initial change this would be my preferred solution compared to the other options so far, although I'm obviously a bit biased here since this means no changes necessary on our side.

My only question is if this solution has any negative effect on the GS side of things? From what @rubidium42 explained, the reason for the change in the first place was that a GS could have issues determining which roadtypes were actually viable to build based on the way we define some of them with both the HIDDEN and TOWN_BUILD flags.

If that was the main issue and this solution solves it for both sides in a good way (which it does from my perspective, at least) then this seems like the ideal fix to me. However I don't really understand the GS side of things so perhaps there is another view here.

[Brickblock1]

I would also prefer this option, while I haven't made a roadset I still think that this option is more flexible and therefore better.

[michicc]

Needs a rebase after the recent CI changes.