Refactor payment method check for gateway method as it should have been written

[wjrosa]

Changes proposed in this Pull Request:

This is a very small refactor fixing is_valid_payment_method_type_for_gateway to how it was supposed to be written. We should have used the attributed variable ($reusable_gateway) so that the first line of the method is not totally ignored.

Testing instructions

Code review is enough. Check if the tests are still passing.

---

• [ ] Covered with tests (or have a good reason not to test in description ☝️)
• [ ] Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• [ ] Tested on mobile (or does not apply)

Post merge

• [ ] Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• [ ] Added the needs docs label (or does not apply)
• [ ] Included this PR in the Release Thread scope (or does not apply)