docs: remove unmaintained and redundant entries

Dead/unmaintained:
- html5lib (no release since 2019)
- fsociety (no commits since 2022)
- optimus (no development since 2023)
- flanker (abandoned by Mailgun)
- imbox (last release Nov 2022)
- green (dormant, pytest dominates)

Redundant/superseded:
- eyed3 (subset of mutagen, GPL)
- odmantic (redundant with beanie)
- audioflux (not established, librosa dominates)
- treq (niche Twisted HTTP client, 606 stars)
- cliff (OpenStack-internal, 260 stars)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>