sim: igen: drop config.h & header checking

sim: igen: drop config.h & header checking

While the configure script was checking for a bunch of headers, only
one of them was conditionally included in the source (unistd.h).  The
rest were always included.  Based on those usage this whole time, we
can reasonably assume that the build also has unistd.h.

All the other files including config.h never actually used any defines
from the header.