pf tests: Do not handle ipfw presence

Initially, it was added to cover a conflicting case of ipfw and pf used
together. But there are more drawbacks than benefits:

• A half of these tests are always skipped. That leads to misunderstanding, while the test suite strives to avoid ambiguous situations.
• Handling enabled ipfw on the test level is tedious, error-prone, and less maintainable.
• CI and similar parties already know how to deal with ipfw for the test suite, like making it open by default. Extra complexity is not needed.

In addition, ipfw+pf use cases are not officially supported.

Reviewed by: kp, markj
Approved by: kp (mentor), markj (mentor)
Differential Revision: https://reviews.freebsd.org/D46655