System DSN you have found in ODBC settings is used as storage of all avast! objects (such as tasks, sessions, mail alerts, scheduler events) except VRDB.
If you dont want to use ODBC you can switch avast! to use XML file instead. If this is the case change value of key Database of INI file from "ODBC" to "XML" and restart all parts of avast! (or computer to be sure
)