Make sure water optimization is disabled for esoteric interactions

This fixes a bug where the generic NB kernel could be called with
a water-water neighborlist for a few special combinations where
no C kernels existed (e.g. switch/shifted plain-cutoff coulomb).
This would typically lead to virtually no nonbonded interactions
being calculated and the simulation crashing rapidly, so it is
unlikely to have affected any results silently, but was noticed
when testing interaction forms.

Change-Id: I634fc4ab78b54281c89333299975e25883dc1f2c