nbody_CPU_AOS: eliminate unnecessary condition

This condition was breaking auto-vectorization and eliminating it makes the
relative error 0.0 when compared to SOA. It does seem strange to me that we
would ever do gravitational pull calculations from one body to itself, but I
can live with it.

Before:

    CPU_AOS:  2820.07 ms =  380.750x10^6 interactions/s (     7.61 GFLOPS) (Rel. error: 3.964103E-02)
    CPU_AOS:  2823.33 ms =  380.311x10^6 interactions/s (     7.61 GFLOPS) (Rel. error: 3.902450E-02)
    CPU_AOS:  2822.05 ms =  380.483x10^6 interactions/s (     7.61 GFLOPS) (Rel. error: 3.063102E-02)

After:

    CPU_AOS:  1360.07 ms =  789.478x10^6 interactions/s (    15.79 GFLOPS) (Rel. error: 0.000000E+00)
    CPU_AOS:  1347.89 ms =  796.608x10^6 interactions/s (    15.93 GFLOPS) (Rel. error: 0.000000E+00)
    CPU_AOS:  1418.86 ms =  756.765x10^6 interactions/s (    15.14 GFLOPS) (Rel. error: 0.000000E+00)

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