iputils.hh: NetmaskTree: Make tree cleanup mandatory.

iputils.hh: NetmaskTree: Make tree cleanup mandatory.

Potentially leaving branches full of unassigned nodes unnecessarily complicates
tree algorithms. Disabling tree cleanup was not used anywhere, except for a unit
test. Note that, after this change, individual branch nodes can still be
unassigned, but not the whole branch. So, when e.g. the left sub-branch of a
node exists, algorithms can rely on the fact that there is at least one assigned
node in there.