Fix: Remove unwanted river artifacts after generation

[SamuXarick]

Motivation / Problem

Artifacts left behind during river generation.

Description

Remove unwanted river artifacts after generation.

Limitations

This requires postponing ModifyDesertZoneAroundRiver to after river generation.

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

[Froozigiusz]

This ain't a generation error, it's just an Oxbow lake 🙃 But I will agree that the current „implementation” 😉 needs some more work and a small dud like the one presented in the screen–shot is undesirable.

[SamuXarick]

With swamps, this is no longer useful.