Codechange: [Emscripten] remove "relative_mode" parameter from -vsdl as it doesn't exist

[TrueBrain]

Motivation / Problem

At the introduction of emscripten ( d15dc9f ) this parameter was set. But I can't find any evidence it was ever implemented.

Description

I suspect during the development of that commit it did exist, but got removed in favour of #ifdef. And then got lost in time. Anyway, we don't deal with relative mode anymore in the global sense, so forget about it.

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

[glx22]

Can't find trace of it either