WPF- data source remove unused connection string

I'm building a WPF application, I created a few extra connections string just for testing reasons. now, i tried removing them from the project settings- but that creates an error in the data source designer:

Error 9 'HelloWorld_APP.Properties.Settings' does not contain a definition for 'HelloWorld11ConnectionString' and no extension method 'HelloWorld11ConnectionString' accepting a first argument of type 'HelloWorld_APP.Properties.Settings' could be found (are you missing a using directive or an assembly reference?) c:\workingfolder\HelloWorld\develop\HelloWorld_app\HelloWorld.designer.cs 8280 97 HelloWorld_APP

so what is the correct way to remove the unused connection strings?

thanks in advance