[RF] Avoid manually deleting RooFit collection in RooWorkspace
[RF] Avoid manually deleting RooFit collection in RooWorkspace

With commit [1], the overhead from the former `safeDeleteList` is gone,
so there is no advantage anymore of doing the deletion of element
manually in an attempt to bypass `safeDeleteList`.

Also, some objects that were on the heap before are moved to the stack.

[1] c46c6d3, "[RF] Remove safeDeleteList functionality of RooAbsCollection"

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root