fix(push-button): remove default label

[markacianfrani]

Brief Description

Removes the default label text 'Push Button'

JIRA Link

ASTRO-3925

Issues and Limitations

We also probably want to deprecate and remove the label prop altogether in favor of slots in the future.

Types of changes

• [x] 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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🦋 Changeset detected

Latest commit: c7a36e2b27a0a56e41397e4f065564f21a3b85f5

The changes in this PR will be included in the next version bump.

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