Issue #3611131: Remove the dead email-based user registration code

Fons Vandammerequested to merge
issue/strava-3611131:3611131-remove-redundant-email-related into 3.0.x

The Strava API no longer returns the athlete's email address, so userCreate(), registerUser() and its helper methods can never work. Remove them along with the now unused UserManager dependencies and the file module dependency that only existed for the user picture download.

Closes #3611131

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