Remove: last remnants of SunOS

[TrueBrain]

Motivation / Problem

We have no evidence SunOS has been tried in the last few years, and the code is awkward enough to just remove.

Description

If someone strongly disagrees, feel free to revert this PR and show it actually still works and is used on SunOS. We are not against having code for SunOS around, but we do against untested code :)

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