[ci] Disable step commit status (#199073)

The number of steps in pull request builds has been causing GitHub API
rate limit issues. In particular, scenarios that cause all steps to fail
have proven to quickly trigger the rate limit.

The disables step statuses on pull requests. We will still have our
required kibana-ci check for the overall build, and the pull request
comment can be used as the source of individual step failures.

.buildkite/pipeline-resource-definitions/kibana-pr.yml

@@ -21,7 +21,6 @@ spec:
       env:
         ELASTIC_PR_COMMENTS_ENABLED: 'true'
         GITHUB_BUILD_COMMIT_STATUS_ENABLED: 'true'
-        GITHUB_STEP_COMMIT_STATUS_ENABLED: 'true'
         GITHUB_BUILD_COMMIT_STATUS_CONTEXT: kibana-ci
       allow_rebuilds: true
       branch_configuration: ''