retire NexGen CPU identification code

retire NexGen CPU identification code

On OpenBSD/i386 we only support Pentium compatible CPUs with an FPU.
The NexGen CPUs probably don't meet this requirement so remove the
small amount of support code to identify this brand of CPU.

The Nx586 "P" models did not include an FPU as they were expected
to be paired with an Nx587 FPU (which was never officially released).

The Nx586 "FP" models did include an FPU, but even these are allegedly
386 clones and not Pentium compatible. According to some websites,
486 instructions were emulated via hypercode. However the 486
instruction emulation was only enough to run applications and wasn't
good enough to run an OS that needs newer 486 and Pentium instructions.

Finally, while our current code references the "Processor Recognition
Application Note", we only implement the half of the documented
algorithm to detect NexGen CPUs. We don't implement the second half
of the algorithm to check for the vendor string "NexGenDriven" on NexGen
CPUs that support the CPUID instruction (like the Nx586-P120 and Nx586-P133).

ok jsg@