Fix 30ae072: when a road type is hidden, towns may not build them even when that flag is set

[rubidium42]

Motivation / Problem

< petern> glx22viaGitHub: Hidden road types should not be buildable by towns either
< petern> I think I had a patch or pr for rhat

Description

Remove logic that hidden roads are buildable by towns and GS, and document this behaviour it with the flags.

Limitations

None.

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