Remove {ATF,PLAIN}_REQUIRE_KERNEL_MODULE macros

[ngie]

Summary

These macros predated the require.kmod (ATF) and required_kmods
(kyua) test metadata and are no longer needed.

Add a hint to freebsd_test_suite/macros.h about the *FEATURE* macros
going away soon. See [this kyua issue][kyua-issue] for more details.

MFC after: 1 month

[kyua-issue]: https://github.com/freebsd/kyua/issues/346

Test Plan

I hand-tested these scenarios on my VM with and without the kernel modules loaded and confirmed that they worked after the change.