refactor(dialog): remove button group under the hood

[markacianfrani]

Brief Description

deprecated button group but dialog is still using it under the hood. this PR removes rux-button-group from rux-dialog

JIRA Link

Related Issue

General Notes

Motivation and Context

Issues and Limitations

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change

Checklist

• [ ] This PR adds or removes a Storybook story.
• [ ] I have added tests to cover my changes.
• [ ] Regressions are passing and/or failures are documented
• [ ] Changes have been checked in evergreen browsers

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Latest commit: bb44bd5fa349839355ff622717158d91751e5eeb

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