[RF] Specify the default RooMinimizer minimizer type in only one place

The default minimizer type was specified both in the RooMinimizer
itself, as well as in the `fitTo` methods. This is not optimal, as one
would have to remember to change it everywhere if the defaults are
changed in RooFit.

In RooStats, there were a few places where the RooMinimizer defaults
were obtained via
`ROOT::Math::MinimizerOptions::DefaultMinimizerType` directly. This
is now done for all RooFit libraries to be consistent.