Revert "forEveryPairBruteForce: became redundant"

This reverts commit 376227894d51c0ab29629e193514f9145ae372e5.

I wasn't thinking that one through. We still have double counting if there
are 2 boxes or less per dimension, because the 'next' and the 'prev' box
map to the same box (the unique neighbour if numBoxes==2; or our own box if
numBoxes==1)