Use the existing fixtures to create users in stead of a migration

The fixtures bundle is already present and a fixture for the users has been created as well:

https://github.com/jmather/symfony-sonata-distribution/blob/master/src/Application/Sonata/UserBundle/DataFixtures/ORM/LoadUserData.php

The fixture should be used in stead of a migration, because those are meant for changes in the database design. Fixtures are meant to be used for cases like this where initial data needs to be created. 

Another advantage is that it doesn't rely on a particular database driver (MySQL in this case) but works with Postgres or SQLite as well.