Remove heuristic group-scheme neighbour-list updates

-1 == nstlist used to trigger the use of an algorithm that did
neighbour searching only when particles had moved far enough that
there might be a need to update the list. This supported obtaining
better energy conservation with the group cut-off scheme. It has been
superseded by the Verlet cut-off scheme.

Part of #1292

Change-Id: I074790c1c5670e874b8b587a1d46e62e2edc8961