Remove a block of "#if 0" code that has been in the file since r1.1, 13

Remove a block of "#if 0" code that has been in the file since r1.1, 13
years ago. This was part of the original reset code port from i386, and
we've never been able to reset amd64 in the way the code tries to do in
the #if 0 block since we don't have a 'PTD' variable in amd64.

I thought to keep this around for diffability with i386, but that's pretty
pointless now as the diff between the two machdep.c files is over 5000
lines already...