[Alerting/API Tests] Add await to objectRemover.removeAll()

[tsullivan]

Summary

Resolves an "unhandled Promise rejection" found for a CI failure for a PR: https://buildkite.com/elastic/kibana-pull-request/builds/181932#018c3b27-e511-4678-9e18-50f90e02c934

2023-12-05 19:03:36 UTC | Unhandled Promise rejection detected:
-- | --
  | 2023-12-05 19:03:36 UTC |  
  | 2023-12-05 19:03:36 UTC | Error: expected 204 "No Content", got 404 "Not Found"
  | 2023-12-05 19:03:36 UTC | at map (object_remover.ts:44:12)
  | 2023-12-05 19:03:36 UTC | at Array.map (<anonymous>)
  | 2023-12-05 19:03:36 UTC | at ObjectRemover.removeAll (object_remover.ts:38:28)
  | 2023-12-05 19:03:36 UTC | at Context.apply (max_queued_actions_circuit_breaker.ts:29:21)
  | 2023-12-05 19:03:36 UTC | at Object.apply (wrap_function.js:73:30)
  | 2023-12-05 19:03:36 UTC | at processTicksAndRejections (node:internal/process/task_queues:95:5)

[ymao1]

LGTM thanks for fixing!

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: ded3846

Failed CI Steps

• Serverless Investigations - Security Solution Cypress Tests #1
• Detection Engine - Security Solution Cypress Tests #6

Test Failures

• [job] [logs] Detection Engine - Security Solution Cypress Tests #6 / Alert user assignment - ESS & Serverless Updating assignees (single alert) adding new assignees via More actions in alerts table adding new assignees via More actions in alerts table
• [job] [logs] Serverless Investigations - Security Solution Cypress Tests #1 / Correlation tab should update timeline after removing eql should update timeline after removing eql

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream