Ref T120, refactoring of modulator related classes

* avoid virtual functions in modulator related classes
* removed unused isDefaultValue()
* using null values (which was not present when class was written)
* removed destructor

src/blackmisc/aviation/transponder.cpp

@@ -54,7 +54,6 @@ namespace BlackMisc
 
         bool CTransponder::validValues() const
         {
-            if (this->isDefaultValue()) return true; // special case
             return CTransponder::isValidTransponderCode(this->m_transponderCode);
         }