squeue: report number of nodes in completing for completing jobs

For some reason squeue was treating completing jobs the same as
pending jobs, and reported the number of nodes as the maximum of
requested nodelist, requested node count or CPUs (divided into nodes?)

This is in contrast to the squeue manpage which explicitly states
that the number of nodes reported for completing jobs should
be only the nodes that are still allocated to the job.

This patch removes the special handling of completing jobs in
src/squeue/print.c:_get_node_cnt(), so that the squeue output for
completing jobs matches documentation. A comment is also added
so that developers looking at the code understand what is going on.