Docs: remove some CSS from .highlight-toolbar def

[julien-deramond]

Description

This PR removes some CSS code used to define .highlight-toolbar:

• border: solid var(--bs-border-color); can be dropped safely since it's the default border color
• border-width: 1px 0; is replaced by border-0 border-top border-bottom
• .btn-clipboard { margin-top: 0; margin-right: 0 } is useless because we already use utilities in this case

bootstrap/site/layouts/shortcodes/example.html

Line 37 in 8a35408

<button type="button" class="btn-clipboard mt-0 me-0" title="Copy to clipboard">

Motivation & Context

Less CSS and more utilities

Type of changes

• Refactoring (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• All new and existing tests passed

Live previews

• https://deploy-preview-37817--twbs-bootstrap.netlify.app/docs/5.3/components/accordion/#example (for example) in light/dark mode for non-regression