Remove user-declared constructors for response class

Removed the following defaulted constructors:

* Default Constructor
* Copy Constructor
* Move Constructor

The defaulted move constructor prevents implicit declaration of
the copy assignment operator, which results in compiler errors
since std::promise requires it.

All 3 user-declared methods removed here are implicitly declared by
the compiler anyway, so no need to default them in the first place.

Relates to cpp-netlib#547

Author: rcdailey

http/src/network/http/v2/client/response.hpp

@@ -60,23 +60,6 @@ class response {
    */
   typedef headers_type::const_iterator const_headers_iterator;
 
-  /**
-   * \brief Constructor.
-   */
-  response() = default;
-
-  /**
-   * \brief Copy constructor.
-   * \param other The other response object.
-   */
-  response(const response &other) = default;
-
-  /**
-   * \brief Move constructor.
-   * \param other The other response object.
-   */
-  response(response &&other) noexcept = default;
-
   /**
    * \brief Swap function.
    * \param other The other response object.