minor speed-up and code clean-up in nbnxn kernels

Clusters without LJ now use the half-LJ instead of the full-LJ
plus Coulomb kernels. This gives a few percent speed-up for water.
Also updated some kernel comments and macros.

Change-Id: Ifa6072561a7042d9d0602ec7d84bc667361951e4
Merge "minor speed-up and code clean-up in nbnxn kernels" into release-4-6