[EuiCard] Various fixes & cleanup from Emotion conversion (#6341)

* [REVERT ME] Repro/testing

* Example of failing cloned CSS

- note missing euiAvatar classes in `emotion-euiCard__icon`

* Fix EuiCard icons not correctly merging `css`

* Fix `EuiCheckableCard` not sending top level `css` prop to the same DOM node as its `className` prop

* [opinionated] Remove unused euiCard modifier classNames

no results in Kibana for `euiCard-` whatsoever, so feels safe to remove

+ clean up types

+ add unit tests for each text alignment

* changelog

* Revert "[REVERT ME] Repro/testing"

This reverts commit 0ab504b.