ci(buildkite): remove the blocked build webhook notification (#13403)

A pipeline-level `notify` webhook can only send to a URL, so it cannot
carry the `X-Buildkite-Token` header the Windmill HTTP route for
`buildkite_notification_webhook` now checks. The blocked build
notification moves to an organisation-level Notification Service webhook
scoped to this pipeline, and the hardcoded steps in the Buildkite
interface no longer include the `notify` block, so this copy drops it to
match.

Signed-off-by: Amir Zarrinkafsh <3339418+nightah@users.noreply.github.com>

.buildkite/pipeline.yml

@@ -14,8 +14,4 @@ steps:
   # Pipeline for all others (tagged commits/local branches/PRs).
   - label: ":pipeline: Setup Pipeline"
     command: ".buildkite/pipeline.sh | buildkite-agent pipeline upload"
-
-notify:
-  - webhook: "<REDACTED WEBHOOK_URL>"
-    if: build.state == "blocked"
 ...