mz: remove deprecated code

[joacoc]

Motivation

This PR removes a temporary fix and warning message. It has been out there for a couple weeks. Now is time to remove the warning.

The intention was to remove it after v0.1.6, we are currently at v0.2.1.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes: