Consider disabling key management feature of Enigmail

We prefer providing a single tool for a single task, unless we have a good reason to do so. With the return of Icedove (#5663 (closed)) we will add Enigmail.

Is it possible to disable its key management features?
Do we want to do that?

Related issues

Related to #5663 (closed)
Related to #9404 (closed)

Original created by @sajolida on 9422 (Redmine)

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