[RF] Don't clone plot variable in `RooAbsReal::plotOn`

The plot variable of the RooPlot is cloned in the implementation of
`RooAbsReal::plotOn`, probably not to disturb it's original state.

This commit suggests to not clone the variable and instead reset its
value to the original value at the end of `plotOn`.

These changes enable the plotting of functions that use the RooFitDriver
internally for evaluation, which is not meant for copying and server
redirection.