docs: remove nix-shell section from contributing.md

[Zaechus]

Pull Request Template

Type of change

• Bug fix
• Feature
• Documentation update

Description

The shell.nix file was removed in f2e8112 but the current CONTRIBUTING.md file still documents it. This commit removes the entire "Development with nix" section.

Checklist

• any anime playing
• bumped version

---

• next, prev and replay work
• -c history and continue work
• -d downloads work
• -s syncplay works
• -q quality works
• -v vlc works
• -e select episode works
• -S select index works
• -r range selection works
• --skip ani-skip works
• --skip-title ani-skip title argument works
• --no-detach no detach works
• --dub and regular (sub) mode both work
• all providers return links (not necessarily on a single anime, use debug mode to confirm)

---

• -h help info is up to date
• Readme is up to date
• Man page is up to date

Additional Testcases

• The safe bet: One Piece
• Episode 0: Saenai Heroine no Sodatekata ♭
• Unicode: Saenai Heroine no Sodatekata ♭
• Non-whole episodes: Tensei shitara slime datta ken (ep. 24.5, ep. 24.9)

[port19x]

nice catch!