Motivation / Problem

CargoSpec::multipliertowngrowth was added in 3d581f4, but no readers have been added since then.
It seems unlikely that readers will be added in the near future.

CargoSpec::multipliertowngrowth is set by cargo property 19.
Cargo property 19 is only implemented in TTDPatch.

The name looks enticing to NewGRF authors, but they are then confused and disappointed when setting it seems to do nothing.

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