[RF] Don't try to un-do modifications to the compute graph in BatchMode

It's to expensive to do for large graphs, too complicated to implement,
an not necessary anymore because also in BatchMode the full computation
graph is cloned.

This also means that the `RooAbsReal` that is evaluated in
`RooAbsReal::getValues()` also needs to be cloned, but this is okay
because the interface is so far only used in unit tests and not
performance critical. It was actually necessary to do the cloning there
to begin with, since the BatchMode was not correctly reverting all
modifications to the computation graph anyway.