How to remove Link To Category from Redactor
I don't think there's an easy way to do this natively in Craft.

Maybe you could pull it off with combination of https://github.com/lindseydiloreto/craft-cpcss and/or https://github.com/lindseydiloreto/craft-cpcjs and manipulate the DOM directly.

Or maybe by going from the Redactor site and listening to their click event.