Makefile: Deorbit fmake support

fmake has been out of the tree for 10 years / 5 major releases now. The
need to bootstrap from it has been gone for at least 6 if not 8
years. While we may still need to bootstrap bmake, we don't need to do
it from fmake, so only retail the infrastructure to update from bmake to
bmake. Retain, for now, the WANT_MAKE_VERSION stuff, though we're always
up to date when building from supported and quasi-supported platforms.

Also remove all the checks to see if .PARSEDIR is defined. It is always
defined and was an early, fail-safe way to tell fmake from bmake during
the transition.

Adjust comments that refer to old fmake and remove those no longer
relevant.

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