[Discover] Remove kbnUrlStateStorage of Discover main state container

[kertal]

Summary

This PR removes the exposed kbnUrlStateStorage property of DiscoverStateContainer. It was just used for testing and can be easily replaced with await new Promise(process.nextTick);

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[jughosta]

Code review only.
LGTM 👍

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 6f619ed

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
discover	421.4KB	421.3KB	-21.0B

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
securitySolution	399	403	+4
total			+6

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
securitySolution	479	483	+4
total			+6

History

• 💚 Build #129459 succeeded 5821cf4

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

cc @kertal