fix(back to top): drop useless CSS rule

[julien-deramond]

Description

This PR drops an unused rule for the back-to-top component. It was useful for the Label inside variant in v5.1.3 with the following markup:

<nav aria-label="Label inside back to top example" class="back-to-top position-static">
  <a href="#top" class="back-to-top-link position-static btn btn-secondary px-3">Back to top</a>
</nav>

It implies some deprecated Sass and CSS variables mentioned into the migration guide.

Types of change

• New feature (non-breaking change which adds functionality)

Live previews

• https://deploy-preview-2340--boosted.netlify.app/docs/5.3/components/back-to-top/
• https://deploy-preview-2340--boosted.netlify.app/docs/5.3/migration/#v533

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[julien-deramond]

FYI I removed "Even if the rendering could still work, it is recommended to only use the versions presented in the documentation." from the migration guide since it's not true anymore and it's rather impossible to test (and we're going to forget it).