Remove size_type note from metab_bayes docs

The roxygen text to `metab_bayes()` has this note:

```r
#' As of summer and fall 2016, a new compilation of any Stan model gives
#' deprecation warnings including \code{typedef 'size_type' locally defined but
#' not used [-Wunused-local-typedefs]}, \code{typedef 'index_range' locally
#' defined but not used [-Wunused-local-typedefs]}, \code{typedef 'index'
#' locally defined but not used [-Wunused-local-typedefs]}, and \code{'void
#' stan::math::set_zero_all_adjoints()' defined but not used
#' [-Wunused-function]}. THESE ARE OKAY. Subsequent runs of the compiled Stan
#' model will be quieter, and the model will work.
```

I suspect these deprecation warnings have disappeared with the completion of #438, but let's check. If so, we can remove this note from the documentation.

Start at the roxygen documentation for metab_bayes() and review the changes from issue #438. Check whether the listed Stan compilation warnings still occur; if they have disappeared, remove the note and rebuild the documentation to confirm the result.

Written by the indexing model from the issue text.