Checkbox: Fix extraneous right hand margin when no label is present

[ashharrison90]

What is this feature?

• removes explicit grid templating and just lets the browser work it out automatically
  • this feels a bit scary 😬
  • have checked storybook in chrome/firefox/safari and seems good
  • have also played around locally and can't see any issues, but this would probably be good to get others to test and confirm

Why do we need this feature?

• so checkboxes without a label don't have extraneous right hand margin

Who is this feature for?

• everyone!

Which issue(s) does this PR fix?:

Fixes #68884

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.

[JoaoSilvaGrafana]

I agree it feels a bit scary but I can't see it changing anything when it is removed, so... lgtm? 👏