Remove verbose parameter from all models

[nicrie]

Initially, I thought having a quick on/off switch to track the progress of dask computations would be useful. However, I realize now that I've never actually used the verbose parameter in any model. The two main reasons are:

1. In its current form, it's not very informative. It shows the progress of each computation process, but users typically don't know how many processes there are, making the information incomplete.
2. You can simply import from dask.diagnostics import ProgressBar and run the computation within a with context for similar progress tracking.

If you're okay with it, @slevang, I'd suggest adding a DeprecationWarning to indicate that we plan to remove this parameter in the future.