That means that the field that you tried to delete has a foreign key realtionship in the serverconfig table.
So, in the mdsagconfig table there is an Id field. If you run this line:
select * from mdsagconfig where serverurl = <your serverurl>
You will see a field for Id. That is the id that is linked to the serverconfig table. You will need to delete the record from the serverconfig table first.
If you have SQL Server, you can use Enterprise Manager to look at table rather than run commands to delete records. It's easier and somewhat safer, incase you run the wrong sql statement
But this line should work for deleting the record from serverconfig:
delete from serverconfig where mdsagconfigid = <id from above statement>
Before doing any of this, backup the database.
Then you should be able to clear the mdsagconfig table that you tried before. Any questions or concerns let us know.