dtrace: remove illumos code from dt_proc.c

The illumos #ifdef's in this file make it harder to read and maintain.
There is little change upstream, and increasing changes for FreeBSD.
Remove the illumos code with unifdef. The only manual changes here
are the #includes and #defines at the top, and removing blank lines.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D41173