Remove assignment operator from octree_path.

There were rule-of-three warnings because octree path declared an
assignment operator but not a copy constructor.  The code for the
assigment operators did exactly the same thing that a default assigment
would do, except there was a special "const" version that cast away
the constness of one member.  However, when I removed this version,
there were no compiler errors, so perhaps it was there just to satisfy
an ancient compiler that could not handle "const" properly with templates.

Change-Id: I3e5e8858d31f0711d49e0281adbfbd7c7b6e381e
Merge topic 'octree-assigment-rule-of-three' into master

5d600bd9 Remove assignment operator from octree_path.