prefer implicitly-defined dtors instead of user dtors with empty bodies. (#685)

prefer implicitly-defined dtors instead of user dtors with empty bodies. (#685)

Not only is this simpler, it avoids clang-tidy complaining about
warning: class * defines a non-default destructor but does not define a copy
constructor, a copy assignment operator, a move constructor or a move
assignment operator [cppcoreguidelines-special-member-functions]