bsd.port.mk: No need to support fmake anymore

There's no need to support fmake anymore. Always assume we can use
bmake's :tA modifier. The ports tree hasn't supported fmake in about a
decade anyway. Simplify here.

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