make_check: Deobit fmake support

We don't need make_check to work in a fmake world anymore (nor have we
in the past decade). Just remove it here.

Note in passing it's been 10 years since we've added a new test here and
maybe we're past the need for this part of the build (or need to revamp
it to include all the features added to bmake since 2016 that the build
system silently depends on).

Sponsored by: Netflix
Reviewed by: brooks
Pull Request: https://github.com/freebsd/freebsd-src/pull/980