Move @kbn/es-query into data plugin - remove unused code

[alexwizp]

Summary

Part of #42885

During moving code from @kbn/es-query/kuery folder into new platform I found some legacy code (in filter_migration folder) which we don't use any more.

In this PR, I just want to double check that we do not need it, and this code can be safely deleted

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]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request
• Commit: 65905f8

[alexwizp]

@elasticmachine merge upstream

[lizozom]

Old code removal LGTM

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 6d1969c

[Bargs]

This code was only used in the first version of kuery where filters would get converted to kuery and added to the query string. Definitely no longer used and safe to delete.