Removing references to PayPal from the Membership Billing page

[kimcoleman]

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• 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:

Our PayPal gateways no longer support showing the Update Billing Information link from the Membership Account page. The Membership Billing page is also now redirected away from since the PayPal gateway(s) do not declate support for the payment_method_updates gateway method.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

• BUG FIX: Removed references to update subscriptions at PayPal from the Membership Billing page.

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

[kimcoleman]

Sites can use a gist like: strangerstudios/pmpro-snippets-library#246 to show a link from the Membership Account > Memberships section instead of the dedicated page.