GLM: Move `mu` out of transformed parameters

[WardBrian]

For our purposes the only difference between a variable being in transformed parameters and having it as a local variable in the model block is whether or not it gets saved as an output. This is different from, say, transformed data, which also has an efficiency benefit.

Especially if we are advertising the use of model.fitted_samples_ and using arviz, we should try to keep as much as possible out of the CmdStan output.