ci: drop redundant .lycheeignore

lychee in ci.yaml already runs with --accept 200,206,403,429, which
treats the bot-blocked responses from Medium, the Palantir blog, and
Forrester as a valid status. The ignore file was belt-and-suspenders
config that gave the false impression those URLs weren't being
checked. The URLs are still structurally verified, we just accept
their 403 response.

This change was prepared by Coder Agents.

Author: DevelopmentCats