Maxadmin 2.2 performance tuning documentation is wrong

As discussed here, the performance tuning section in MaxScale 2.2 documentation makes references to maxadmin statistics outputs that are no longer present and hints at adjusting parameters that no longer matter.

Suggest removing the outdated references and replacing them with up-to-date guidelines as to how performance of MaxScale should be observed, which metrics are relevant and how MaxScale should be tuned (if at all) to best take advantage of the OS resources.