Delete unused buildkite pipeline/webhook

[steveri]

The buildkite CI test for this repo, which used to be here:
https://buildkite.com/stanford-aha/dragonphy2
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/StanfordVLSI/dragonphy2/tree/master/.buildkite)
and can be used if/when you might want to reinstate the pipeline at some future time. But the old webhook
ad33aab8
no longer points to anything, and it can safely be deleted
(see https://github.com/StanfordVLSI/dragonphy2/settings/hooks).