The variable "minslptime" is pointless and always has been, ever since its
introduction in r83366. (At that time, this code appeared in vm/vm_glue.c,
because vm/vm_swapout.c did not exist.) When the FOREACH_THREAD loop
completes, we know that the sleep time for every thread is above whichever
threshold is being applied.

Reviewed by: kib
X-MFC with: r327354