Because of a bug in sequelize, the hardcoded name of the database in config.json was used with mysql/mariadb databases. This commit removes all content from config.json, so environment variables are always used. The file itself must still exist, as HedgeDoc does not start without a config file.

For details see https://github.com/hedgedoc/container/issues/136#issuecomment-747072862Signed-off-by: David Mehren <git@herrmehren.de>