Apply Rule of Zero to disulfide energy neighbor iterators (#689)
- Remove user-declared trivial (defaulted) destructors from the six
disulfide neighbor iterator classes; the implicit virtual destructor
inherited from `ResidueNeighborIterator` /
`ResidueNeighborConstIterator` does the same job.
- Modernize the old-style C++03 copy-assignment prevention (private
undefined declaration) to a public `= delete` for the derived-type
`operator=`. Behavior is unchanged: derived-derived assignment is still
forbidden so all assignments flow through the polymorphic
`operator=(BaseType const &)` override that downcasts and copies all
members.