OK there were basically 2 ways to attach them, with clips or with no hardware. And no hardware attachment means they had to use glue.
Now, since you actually had hardware attaching it, I have to assume it's simply stuck there now with Thermal Interface Material. This stuff is usually a type of putty, it's sticky and softens when warm. You can usually get it off the chip by applying a small amount of lifting force to break it free.
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