Fix DataHandle::get() for non-collection types and remove dead code

[jmcarcell]

BEGINRELEASENOTES

• Fix DataHandle::get() for non-collection types and remove dead code. A reinterpret_cast was changed to static_cast in Use std::unique_ptr<podio::CollectionBase> to save collections in the store and fix leak in the Writer #250 that made it fail at compile time for non-collection types. Now the static_cast is properly wrapped around an if constexpr and code around it has been deleted since it looked impossible to trigger.

ENDRELEASENOTES

[jmcarcell]

k4Gen needs this to compile. Changes needed for k4RecCalorimeter after this are here: HEP-FCC/k4RecCalorimeter#123.