Deleting Mosaico template should also delete corresponding message template

Problem

• After deletion of Mosaico template corresponding message templates are not deleted. These message templates are marked as 'is_reserved=1' hence, it can't be deleted using Message template UI. This creates unnecessary confusion as CiviCRM still shows the message templates that have been deleted in Mosaico.

Solution

• When Mosaico template is deleted, it should also delete corresponding template.

Let me know if I am missing something or if this is a bad idea.