Sorry but I remain skeptical of your conclusion. Have you hit Ctrl+D on those books since you deleted the tags and restarted? The only way my plugin could add those tags is if you modified it.
I think what is more likely is that you had not hit Ctrl+D before installing the Goodreads plugin, you saw the tags appear, and you conclude it was the Goodreads plugin that added them. You have other metadata download plugins installed with Calibre that run when you hit Ctrl+D that are capable of bringing in tags, and I am positive it was one of those that created them.
You say you have removed the Goodreads plugin, and deleted the tags. Hit ctrl+D again on that book. The tags you saw will appear again provided you have not been fiddling with the other plugin configurations to either disable or skip downloading tags.
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