Codechange: Use std::filesystem::remove/rename in settingsgen.

[PeterN]

Motivation / Problem

Use of unlink/rename in settingsgen.cpp which requires system-type includes.

Description

Use std::filesystem::remove/std::filesystem::rename instead.

std::filesystem::rename specifies that renaming to an existing file will replace it, so an additional remove for Windows is not necessary.

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, 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

[rubidium42]

With or without failing for being unable to remove the temporary file, it's fine by me.