[RF][HS3] Remove redundant check in variable export

The check for already added variables was redundant, because in the
`exportObject()` function we already check if a given argument was
already exported, tracking this in a standard map.

This drastically reduces performance and memory usage, because the
lookup in the JSON itself was very slow and memory hungry, because of
some caching that nlohmann-json tries to do when looking up in array
collections.

Author: guitargeek

roofit/hs3/src/RooJSONFactoryWSTool.cxx

@@ -1016,11 +1016,6 @@ void RooJSONFactoryWSTool::exportVariable(const RooAbsArg *v, JSONNode &node)
       return;
    }
 
-   // this variable was already exported
-   if (findNamedChild(node, v->GetName())) {
-      return;
-   }
-
    JSONNode &var = appendNamedChild(node, v->GetName());
 
    if (cv) {