[RDF] Fix ROOT-9898

Before this patch: he destructor of a derived node type destroyed
its shared_ptr to the previous node. The destructor of the base node
type, called _afterwards_, called fLoopManager->Deregister(this).
In the special case in which the previous node was precisely
RLoopManager and no other RInterfaces or nodes held a shared_ptr to
it, the RLoopManager was being destructed (in the dtor of the derived class)
just before that same node called Deregister on it (in the dtor of the
base class).

After this patch: the derived node types are responsible for calling
Deregister, before destroying the shared_ptr to previous node.

This fixes the use after delete reported in ROOT-9898.