The login information is usually cached by the SVN client on your local machine, outside of SQL Source Control. (The window you see in SQL Source Control is because SQL Source Control is being asked for credentials by SVN).
The credentials are stored in the following location on your machine typically:
If you rename the "auth" folder, it will prompt you again for credentials the next time you attempt to link. However, this will affect your existing linked databases, so beware before performing this step. There is also a readme.txt file contained in that same directory, that contains some instructions as well that may be helpful.
Red Gate Software Ltd.