How can I deinstall V5, when the migration to V7 was successful?

You can filter by "acym" in Joomla extension manager and uninstall all components, modules and plugins with version lower than 7.x

And can I also delete in the database the tables with acymailing prefix?

When you uninstall AcyMailing 5, at the end of uninstall process, you will see an SQL query which you can execute to remove all version5 related DB tables