Eliminated pick_nbnxn_resources

Eliminated pick_nbnxn_resources

The data about which GPUs on a node will be used by the PP tasks on it
is stored when the dev_use array is filled, either by user selection
or the automated per-rank round-robin assignment in
gmx_select_rank_gpu_ids. It is filled so that the PP intra-node rank
IDs will index into it, so aspects of the task assignment can be seen
as already complete. The code in pick_nbnxn_resources makes the final
action of indexing with the intra-node rank ID, then handles the
initialization of the selected GPUs.

Note that nothing meaningful has happened since the the call to
gmx_select_rank_gpu_ids. mdrun has done some checks, called
init_forcerec (which then set up a lot of unrelated things), and then
called init_nb_verlet. The actual task assignment (selecting the GPU
for this task) is where it always was (ie implicitly somewhere between
filling the dev_use array, and the act of actually indexing into it),
and future design options are still as open as they ever were.

Change-Id: Iac908bb74dbec20edaecf6fbfcd18b87179e7aba