Revert "Do not instantiate template ctors to make it take an ioctortype arg."

This reverts commit 223f631d327cd08540263c985d127cebc3255f9f.
Philippe and I violently disagree on whether template <typename T> Klass(T) should be taken into account when matching the RootIOCtor type.
I find this intricate, close to random (because it depends on what rootcling sees or whether the instantiation has been moved to a source file), non-obvious for such an explicit request.
Philippe finds it handy to invade / kidnap "unintrusively" existing templated c'tor signatures.
I request an explicit c'tor overload.
Philippe thinks that's too much of a requirement; he could not find a use case for the templated c'tor, though.