Fix compatibility with Pay By Check

[dparker1005]

All Submissions:

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

Changes proposed in this Pull Request:

Avoiding calling pmpro_getLevelAtCheckout() prevents issues where PBC would always assign level ID 1, even when a gift level isn't being purchased at checkout. The other changes in this PR are for simplification.

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] 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.