[User management] delete avatar endpoint missing

[sholzer]

We need an endpoint to delete the user's avatar image.

example from classic-ui:

[github-actions]

Thanks a lot for reporting the issue. We did not consider the issue as "Pimcore:Priority", "Pimcore:ToDo" or "Pimcore:Backlog", so we're not going to work on that anytime soon. Please create a pull request to fix the issue if this is a bug report. We'll then review it as quickly as possible. If you're interested in contributing a feature, please contact us first here before creating a pull request. We'll then decide whether we'd accept it or not. Thanks for your understanding.