Avoid using `RooAbsArg::replaceServer()` in xroofit

I explain in this PR to upstream ROOT why the RooAbsArg::replaceServer() function is dangerous and should not be used. Using it will soon result in a warning: https://github.com/root-project/root/pull/12040

One needs to find all the usages of RooAbsArg::replaceServer() in xroofit and replace them with the safer RooAbsArg::redirectServer(). If this replacement is not possible or meaningful, I need to go back to the drawing board.