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Interesting question!
You could go through the effort to search, if they have a new site, which would display your appreciation for their work (not much effort after all).
If you can't find a new link, just remove it, but leave the part the link was wrapped around, as this is usually the name of the developer.
You will often find, that if they place a link, designers request it not to be removed, but having a broken link doesn't look good for your site. Therefore I regard the above as a logic conclusion.
Just my 5 c's though....