Remove unused radiogroup styles

[aurorascharff]

Describe your changes

After refactoring the radio group option to a separate component a while back, unused styles for the radio group option were left behind in radiogroup.css. Remove them. Update README.md for MdRadioGroup.

Checklist before requesting a review

• [x] I have performed a self-review and test of my code
• [x] I have added label to the PR (major, minor or patch)
• [ ] If new component: Is story for component created in stories-folder?
• [ ] If new component: Is tsx-file import added to packages/react/index.tsx?
• [ ] If new component: Is css-file added to packages/css/index.css?

[github-actions[bot]]

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