cleanup integration test

[mshustov]

Summary

Closes #36256
after #35297 we can use setup/start contracts in tests and don't need to declare a separate plugin for this

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• [ ] This was checked for cross-browser compatibility, including a check against IE11
• [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• [ ] Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• [ ] This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[elasticmachine]

Pinging @elastic/kibana-platform

[joshdover]

LGTM, tests are much clearer now