Codechange: Introduce FioRemove() to remove files.

[PeterN]

Motivation / Problem

unlink is used in various places, but depending on platform, might actually be a macro to _wfunlink with OTTD2FS being called too.

Description

New function FioRemove() handles OTTD2FS conversion (which already does nothing on platforms that don't need it), and uses std::filesystem::remove instead of unlink, all in one location.

Limitations

Adds a debug message which is perhaps not necessary.

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, game_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