fixes #2219: remove the less documented domain field

[janstarke]

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process?
• If submitting code/script changes, have you verified all tests pass locally using make test?
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes?
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed.
• Have you added an entry to the CHANGELOG.next.md?

[cla-checker-service]

💚 CLA has been signed

[ebeahan]

@janstarke thanks for filing the issue in #2119 and submitting the fix!

Would you please sign the Elastic CLA using the email address you used for committing this fix?

[janstarke]

I already did. Do I need to reopen a pull request after signing the agreement?

[ebeahan]

@janstarke Thanks! I see the CLA check is now ✅

I'm going to add an entry to the CHANGELOG.next, both otherwise LGTM.