Remove obsolete addition of MACHINE_CPUARCH

In the past, we would add symbolic links for MACHINE_CPUARCH when it differed
from MACHINE. This was for pc98 only, however. All other architectures didn't
need this and it was really due to pc98 pulling from i386 rather than something
more intrinsic. At the time, we had the split we did to mimic what NetBSD did
for its 68k ports where many different kernels were possible for the same
architecture. Since then, both projects have moved away from this convention to
having a more generic MACHINE for each architecture. FreeBSD's new arm64/aarch64
breaks this old notion and so was an exception to the rule. So, we no longer
need to create this link for any old machine or any new machine, delete it
entirely.

Differential Revision: https://reviews.freebsd.org/D22246