[C++][Pistache] Fixed #2643 (#2653)

* Fixed #2643

Refactored to/from json functions to use universal object serialization method.

* Code review found incorrect indenting and I forgot to remove unused mustache files.

* Removed helpers class because it is not needed anymore.

* Removed helpers package from docs.

* Reverted helper class removal.

samples/server/petstore/cpp-pistache/model/User.h

@@ -19,9 +19,8 @@
 #define User_H_
 
 
-#include "ModelBase.h"
-
 #include <string>
+#include <nlohmann/json.hpp>
 
 namespace org {
 namespace openapitools {
@@ -32,19 +31,12 @@ namespace model {
 /// A User who is purchasing from the pet store
 /// </summary>
 class  User
-    : public ModelBase
 {
 public:
     User();
     virtual ~User();
 
-    /////////////////////////////////////////////
-    /// ModelBase overrides
-
-    void validate() override;
-
-    nlohmann::json toJson() const override;
-    void fromJson(const nlohmann::json& json) override;
+    void validate();
 
     /////////////////////////////////////////////
     /// User members
@@ -106,6 +98,8 @@ public:
     bool userStatusIsSet() const;
     void unsetUserStatus();
 
+    friend void to_json(nlohmann::json& j, const User& o);
+    friend void from_json(const nlohmann::json& j, User& o);
 protected:
     int64_t m_Id;
     bool m_IdIsSet;