Revert "implement new ai frontend gradient button variant"

[thijsoo]

Context

• Reverts implement new ai frontend gradient button variant #22579

Summary

This PR can be summarized in the following changelog entry:

• Reverts a not needed commit that broke the sidebar menu in the settings page.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Delete the introductions user meta (yoast_wpseo_introductions) to trigger the insights modal again.
• Go to one of yoast admin pages like the settings page.
• Smoke test the insights introduction modal gradient button.
• Check the the focus is on the gradient button.
• Also confirm that the settings page has its sidebar menu back

[leonidasmi]

CR + Acceptance is ✅