## Summary

This PR is part of EUI's ongoing Emotion migration. We are actively
deprecating/removing Sass mixins and variables that are
component-specific.

In this case, the `@mixin euiPanel` **only** sets `flex-grow: 1;` as its
default CSS and creates a bunch of modifier classes, none of which were
being used in Kibana - hence various removals (see git commits for more
specifics).

### QA

- [x] Checked Search Profiler for visual regressions
- [x] Checked Visualize Library > TSVB for visual regressions

### Checklist

- [x] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [x] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

(cherry picked from commit cf72f3e)

# Backport

This will backport the following commits from `main` to `8.x`:
- [Remove Sass `@euiPanel` mixin usage
(#194646)](#194646)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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Co-authored-by: Cee Chen <549407+cee-chen@users.noreply.github.com>