[ADT] Achieve the "Rule of Zero" in DGNode (#165190)
This patch achieves the "Rule of Zero" in DGNode by removing the
copy/move constructors and copy/move assignment operators.
Note that the code being deleted does a couple of unusual things that
are most likely oversight:
- The copy constructor with "explicit" is highly unusual. This means
that we allow "DGNode<N, E> A(B);" but disallow
"DGNode<N, E> A = B;".
- The move assignment operator with const r-value reference is also
unusual, especially given that the move constructor is correctly
implemented.
