FamilyManager.RemoveParameter() method is supposed to delete parameters in the family document, as its name may suggest. However, it's not so true. Those builtin parameters are out of the scope.
"It is a builtIn parameter which cannot be removed."
Obviously, it would have made things a bit clearer to name the method as RemoveNoneBuiltinParameter(). The good thing is that it's kind of consistency between the API and its product feature in this case. Revit users cannot remove builtin family parameters, so cannot the API either. Good or bad!
Once again, Revit API behaves very different from other APIs such as Windows' and AutoCAD's. People can do pretty whatever they want there. The only limitation is that the program or addin have to run in that environment. Maybe that is the reason why those platforms are so popular and powerful.