Here are the steps that should work for you:
This steps are copied from pocketlogic's answer at StackOverflow
- Quit Xcode, and make a backup of your project file ( projectName.xcodeproj ).
- In Finder, right-click on your project file and select Show Package Contents.
- Right-click on the project.xcworkspace file, and select Show Package Contents.
- In the folder named xcshareddata, there is a file with the extension .xccheckout (it should have the same name as your main project file, but with the .xccheckout extension). Open the .xccheckout file in your favorite plain-text editor - this is a plist.
- Find the IDESourceControlProjectWCConfigurations key, and look for the <dict> that has the name of the repository that you want to remove. In the same \ there is a key IDESourceControlWCCIdentifierKey whose value contains an i