Remove harmful dtor definition

The definition of the dtor is not necessary and actually harmful since
the definition of a class's implicit copy constructor is deprecated if
that class has a user-declared constructor as of C++11.  Compilers can
warn about this with `-Wdeprecated`.

include/cxxopts.hpp

@@ -1302,8 +1302,6 @@ template <>
 class standard_value<bool> : public abstract_value<bool>
 {
   public:
-  ~standard_value() override = default;
-
   standard_value()
   {
     set_default_and_implicit();