Remove all traces of using Gravatar without transport security

Summary:
This was effectively disabled in the UI already, but code that doesn't
exist also can't accidentally be reactivated :)

Reviewers: #kde_pim, mlaurent

Reviewed By: #kde_pim, mlaurent

Subscribers: mlaurent

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D7408