[RF] Remove `RooGenFunction` and `RooMultiGenFunction`

The `RooGenFunction` was only a lightweight adaptor that exports a
RooAbsReal as a `ROOT::Math::IGenFunction`. The same can be easily
achieved with the generic `ROOT::Math::Functor1D`, so in the spirit of
not duplicating interfaces, the `RooGenFunction` is removed.

The same goes for the `RooMultiGenFunction`.