Motivation / Problem

Effectively dead code since 8a2da49.
Out-of-date documentation since c921f6d.

Description

Remove: the concept of UnknownGRFs

These were filled with "<Unknown>" (before 8a2da49) and later their name would get filled via UDP requests to the server.
These UDP packets do not exist anymore, so they will always remain "<Unknown>".
Remove that logic and just use the generic translated error GRF UNKNOWN string instead.

And update the documentation of the GameInfo packet with the Game Script fields / for version 5.

Limitations

None. Well, the NewGRFs will now always be called "Unknown" (or whatever the translation is). Later updates to the Game Info protocol and the GC should remedy this problem in the near future (if all goes to plan).

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 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