chore: fully remove deprecated charts theming exports

[nickofthyme]

Summary

Removes all exports and build processes related to chart theming that were deprecated in #7572.

Following merging of elastic/kibana#179503, the EUI_SPARKLINE_THEME_PARTIAL is no longer used in kibana and can be fully removed. Usages of EUI_CHARTS_THEME_DARK and EUI_CHARTS_THEME_LIGHT were removed in elastic/kibana#170914.

BREAKING CHANGE: Removed EUI_CHARTS_THEME_DARK, EUI_CHARTS_THEME_LIGHT and EUI_SPARKLINE_THEME_PARTIAL exports.

[github-actions]

This PR contains breaking changes. The opener of this pull request is asked to perform the following due diligence steps below, to assist EUI in our next Kibana upgrade:

• If this PR contains prop/API changes:
  • Search through Kibana for <EuiComponent usages (example search)
  • In the PR description or in a PR comment, include a count or list with the number of component usages in Kibana that will need to be updated (if that amount is "none", include that information as well)
• If this PR contains CSS changes:
  • Search through Kibana for the changed EUI selectors, e.g. .euiComponent (example search)
  • In the PR description or in a PR comment, include a count or list with the number of custom CSS overrides in Kibana that will need to be updated (if that amount is "none", include that information as well)
• 🔍 Tip: When searching through Kibana, consider excluding **/target, **/*.snap, **/*.storyshot files to reduce noise and only look at source code usages
• ⚠️ For extremely risky changes, the EUI team should potentially consider the following precautions:
  • Using a pre-release release candidate to test Kibana CI ahead of time
  • Using kibana-a-la-carte for manual QA, and to give other Kibana teams a staging server to quickly test against

Log | Bot Usage

[cee-chen]

We have this scheduled for June as a heads up Nick (#1469) - are you cool keeping this PR open until then?

[nickofthyme]

We have this scheduled for June as a heads up Nick (#1469) - are you cool keeping this PR open until then?

Yeah absolutely, just want to get it off my plate. I'll keep it as a draft PR until then, I also updated the meta issue to point to this PR.

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_7682/
• Storybook: https://eui.elastic.co/pr_7682/storybook

[elasticmachine]

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