eliminate _threaded and _openmp variants, make stock variants OpenMP

If we build without OpenMP or use OMP_NUM_THREADS=1, you can see what
performance is like with a single thread.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody.cu: remove outdated build instructions from header

Signed-off-by: Steven Noonan <steven@uplinklabs.net>