Delete unused buildkite pipeline/webhook

[steveri]

The buildkite CI test for this repo, which used to be here: https://buildkite.com/stanford-aha/svreal 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 in this repo: https://github.com/sgherbst/svreal/tree/master/.buildkite/pipeline.yml and can be used if/when you might want to reinstate the pipeline at some future time. But the old webhook f78f1e38 no longer points to anything, and it can safely be deleted (see https://github.com/sgherbst/svreal/settings/hooks).