You cannot remove any assets, missing or not, by deleting them from the config.txt file.
One possible remedy is to run a database repair. From the TANE Launcher window, open the Developer menu and select Rebuild Database. How long this will take will depend on the size of your database and the speed of your system.
Of course, if the unknown items are unknown because they are not on the DLS then this will not help you. What did you recently install that is reporting assets as missing/unknown?
