[BUGFIXING] #39 Discount code ignored

[MaximilianoRicoTabo]

• Bail if discount code is applied via URL.
• Remove payment plan card if discount code is applied via AJAX.

All Submissions:

• Have you followed the Contributing guideline?
• 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:

Resolves #39

How to test the changes in this Pull Request:

1. Apply code via URL
2. Observe that payment plans box isn't rendered and price applied is correct.
   3.Apply discount code via AJAX
3. Observe that in case code is correct the PP box is removed.

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

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