Delete unused buildkite pipeline/webhook

[steveri]

The buildkite CI test for this repo, which used to be here: https://buildkite.com/stanford-aha/anasymod appears to be long dead and maybe never used at all. So, in the interest of efficiency, I am deleting the pipeline from the buildkite repo. The pipeline steps still exist (https://github.com/sgherbst/anasymod/tree/master/.buildkite) and can be used if/when you might want to reinstate the pipeline at some future time. But the old webhook 199e2585 no longer points to anything, and it can safely be deleted (see https://github.com/sgherbst/anasymod/settings/hooks).