Unify nbnxn kernel dispatchers

Unify nbnxn kernel dispatchers

Reduced code duplication by merging the three nbnxn kernel dispatch
functions for C reference, simd_4xn and simd_2xnn into a single
dispatcher. This also removes the implementation details from
sim_util.cpp.

More reorganization could be done, but is not included here to
minimize the size of this change.

The original goal of this change was to conditionally compile all
files in the simd_4xn and simd_2xnn directory, but currently we
do not have the conditions available at configure time.

Change-Id: I41f7ae96d267a12cf6024866b59bb0cda7e1cd2f