Remove serialization exports of Atom class.

Atom class is not involved in any inheritance hierarchy.
It does not need to be explicitly registered for serialization.

Author: pavoljuhas

src/diffpy/srreal/AtomicStructureAdapter.cpp

@@ -305,7 +305,6 @@ const Atom& AtomicStructureAdapter::operator[](int idx) const
 // Serialization -------------------------------------------------------------
 
 DIFFPY_INSTANTIATE_SERIALIZATION(diffpy::srreal::Atom)
-BOOST_CLASS_EXPORT_IMPLEMENT(diffpy::srreal::Atom)
 DIFFPY_INSTANTIATE_SERIALIZATION(diffpy::srreal::AtomicStructureAdapter)
 BOOST_CLASS_EXPORT_IMPLEMENT(diffpy::srreal::AtomicStructureAdapter)
 

src/diffpy/srreal/AtomicStructureAdapter.hpp

@@ -173,7 +173,6 @@ bool operator!=(
 
 // Serialization -------------------------------------------------------------
 
-BOOST_CLASS_EXPORT_KEY(diffpy::srreal::Atom)
 BOOST_CLASS_EXPORT_KEY(diffpy::srreal::AtomicStructureAdapter)
 
 #endif  // ATOMICSTRUCTUREADAPTER_HPP_INCLUDED