Simplify signup style

[kimcoleman]

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• Have you followed the Contributing guidelines?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Moving the top and bottom border out of this so sites have more flexibility to add styling to the Signup blocks in the forthcoming Advanced Signups Add On.

Changelog entry

• BUG FIX: Removed custom styling applied to Signup Shortcode to allow more flexible styling in block editor directly.

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

[RachelRVasquez]

Tested locally. Looks good!