[backpack-web]: Revert CSS layers due to incompatibility with supported browsers

[olliecurtis]

This change reverts our recent efforts with CSS layers #3119 as we found that it is incompatible with Safari versions 15.4 and below which is below our current supported range, and causes issues with functionality in use.

We will re-review the approach post this reversion on how we can progress forward with mitigation CSS issues

Remember to include the following changes:

• Ensure the PR title includes the name of the component you are changing so it's clear in the release notes for consumers of the changes in the version e.g [KOA-123][BpkButton] Updating the colour
• README.md (If you have created a new component)
• Component README.md
• Tests
• Storybook examples created/updated
• For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here

[github-actions]

Visit https://backpack.github.io/storybook-prs/3210 to see this build running in a browser.

[alakbarz]

Skimming over the 250 changes, it looks good to me.