Remove validate-ci-baseline once no pre-trim PR is open

#19514 adds `validate-ci-baseline`, a do-nothing job whose only purpose is to be a required context that the old matrix never produced, so a PR holding green checks from before the trim cannot merge on them.

It has no value once every open PR has run against the reduced matrix. Delete the job from `bot.yml` and the context from `.asf.yaml` at that point.

Part of #19524.

First verify that no open PR still predates the matrix trim. Then inspect bot.yml and .asf.yaml, remove validate-ci-baseline and its required context, and confirm the CI configuration no longer references either one.

Written by the indexing model from the issue text.