treeiter—Delete TreeValueProxy copy-assign/Avoid warning

The user-provided, unimplemented operator=(TreeValueProxy const&) existed to prevent the class being copied, but since a long time C++ provides a better way to do that: declaring the operator as deleted. Doing this also avoids warnings about the implicitly declared default copy constructor, which is deprecated due to the previously user-given operator=, and since the latter is now deleted the warnings are avoided.

resolves #94 (closed)