awtpman: system multiplier removed in favor of model rate.

awgtpman used the system multiplier to guess at the model rate: the rate was either 2k * multiplier or 16k * multiplier.

This MR also gets rid of some internal distinctions between slow and fast models. The distinction still has to be made externally by clients, since the client API is untouched.

Some obsolete defines and some dead code are also deleted.