2.0 more deprecation removals

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

A few more deprecation removals:

• model.format
• vb.stan_variable now always returns an np array, and returns the variational sample by default
• mle.stan_variable now always returns an np array

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):
Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[amas0]

Looks good to me -- one minor change we can make now that the typing of stan_variables has been simplified.