AMP Compatibility: Remove social share CSS

[philipjohn]

Changes proposed in this Pull Request:

The CSS for the share icons triggers a "illegal_css_at_rule" AMP validation error because of the use of '-moz-document'. Given AMP uses the amp-social-share component, this CSS isn't needed at all. This change tells Jetpack not to enqueue the CSS at all when an AMP page is being viewed.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

This builds upon existing AMP compatibility fixes.

Testing instructions:

1. Enable the AMP plugin in Transitional mode
2. Enable social sharing icons in Jetpack
3. View a post, and observe the illegal_css_at_rule validation error
4. Apply the PR, and refresh the post to see the validation passes

Proposed changelog entry for your changes:

• AMP compatibility fix for sharing button CSS

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello philipjohn! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D30150-code before merging this PR. Thank you!

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

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[westonruter]

This brings up something else: shouldn't the social sharing buttons in AMP get settled the same way as they do on the non-AMP page? The amp-social-share elements are styleable.

[jeherve]

This works in my tests. 👍