Chore: Move SCSS mixins to be where they're used

[ashharrison90]

What is this feature?

• consolidates SCSS mixins to be located where they're used
  • removes a few unused mixins
  • all the remaining mixins are only used in one place
  • move them to be where they're used for easier cleanup later

Why do we need this feature?

• clean up old deprecated SCSS 🧹

Who is this feature for?

• grafana devs

Which issue(s) does this PR fix?:

For #64129

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.