I would check where you got the 3rd party content you've added - could it be expecting you to have also downloaded another piece of 3rd party content? See if the author left any notes about that, or even if they didn't leave any notes, see if there's something else they offer that you didn't decide to use - the author may have accidentally created an unintended dependency to some of their other material.
Before deleting the files one by one, in the Tools menu within Hero Lab, choose "Manage Third Party Updates" - that will allow you to delete them as packages, instead of finding the individual pieces of each package.
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