Remove KUI usage in opensearch_dashboards_legacy

[joshuarrrr]

The opensearch_dashboards_legacy plugin uses a single .kui* namespaced class. See https://github.com/search?q=repo%3Aopensearch-project%2FOpenSearch-Dashboards+kui+NOT+trackUI+path%3A%2F%5Esrc%5C%2Fplugins%5C%2Fopensearch_dashboards_legacy%2F&type=code

To resolve

Investigate and inspect the applied CSS of that class and the rendered component. If necessary:

1. Document CSS rules provided by these classes
2. Replace these components with OUI components

Note that opensearch_dashboards_legacy still needs to be rewritten from Angular to React - #1560

It may make sense to just de-angularize first and replace with the OUI react components.