Remove duplicate fields of rollup based index patterns

[kertal]

Summary

Fixes #36297 introduced by #28762. It caused the fields of rollup based index patterns to be duplicated. After the merge of rollup field data with index field data, the rollup field data was appended a second time.

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
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[elasticmachine]

Pinging @elastic/kibana-app

[cjcenizal]

Since this is a bugfix, is this worth backporting to 7.2?

[cjcenizal]

Nice work debugging this! Tested locally, code LGTM.

[cjcenizal]

@kertal Could you add Fixes {URL for original issue} to the PR description to associate the PR with the original issue? This will also auto-close the issue once this is merged.

[elasticmachine]

💚 Build Succeeded

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

[kertal]

@kertal Could you add Fixes {URL for original issue} to the PR description to associate the PR with the original issue? This will also auto-close the issue once this is merged.

of course, done

[kertal]

@cjcenizal what do you think about fixing duplicate data on conversion? I think rollup support is still experimental, if yes, I don't think it's necessary to fix the data

[cjcenizal]

@kertal What's the alternative for fixing a rollup index pattern that's broken due to this bug? Recreating the index pattern and reassociating it with the dependent saved searches, visualizations, and dashboards? I could be wrong but this seems like a lot of work for the user. If so, then personally I think we owe it to the user to create the conversion. This way we can definitively state in our release notes that this bug is fixed in a specific version, since users upgrading to that version will have their index patterns magically healed.

[kertal]

I can recommend going to elastic meetups. there was no bug visible in the presentation of @xeraa tonight at Vienna's elastic meetup. He was using 7.1. Going back a little in the history of this bug, I found out, that it was part of a #28762, which will be part of 7.2. there's no conversion necessary, since the bug didn't make it into production, and we won't let him escape

[cjcenizal]

🎉 Awesome!