Remove obsolete addition of MACHINE_CPUARCH

[imp]

Summary

When MACHINE != MACHINE_CPUARCH, we added the latter to the ILINKS that we
created for module building. This was needed for pc98. The thinking at the time
was that we might have additional architectures where we'd have multiple
MACHINEs for a defined MACHINE_CPUARCH, as NetBSD did for all its 68k
architectures. However, FreeBSD never went that route, and NetBSD has relegated
this practicec to legacy status. Since we're unlikely to have an any more
architectures like this, and since arm64/aarch64 sets the new standard for the
project, remove this legacy of our past now that pc98 has been retired.