Remove assignment operator from octree_iterator.

This is a follow-up to gerrit I3e5e8858d31f0711d49e0281adbfbd7c7b6e381e.
In that commit, I removed the assignment operator from octree_path
because it looked as though the only reason for a custom-defined
operator was to work around some const-related bugs in Visual Studio 6.
The octree_iterator class is a subclass of octree_path that had exactly
the same issues.

Change-Id: I15e7622e7c8bde139770e7bf6ff058360f0335fd
Merge topic 'octree-assignment-rule-of-three' into master

d2785d14 Remove assignment operator from octree_iterator.