[ui][registration][template] remove registered users migration emails

Part of #569

beat/web/ui/registration/templates/registration/mail.migration_10_accounts.message.txt

-Dear {{ user.first_name }},
-
-The admin of the BEAT platform would like to inform you of a security update
-to your account used under username {{ user.username}}
-
-From now on you will need to be supervised by a known supervisor.
-
-To comply with this new rule, please go to your account settings and under
-Account Management click on "Add a supervisor" and validate it with a known
-supervisor.
-
-Failing to do so, your account will be blocked after a few weeks! (you
-can however still re-activate it through an account reactivation procedure)
-
-If you are having problems to revalidate your account, contact a member of our
-support staff at {{ prefix }}{% url 'contact' %}.
-
-BEAT Administrators at the Idiap Research Institute

beat/web/ui/registration/templates/registration/mail.migration_10_accounts.subject.txt

-Major Account Update - Supervision Required (BEAT platform)