Alan, it sounds like this is "cutback" adhesive, more in line with what Tom had mentioned.
There's not much that I'm aware of that can be done to remove cutback short of backbreaking labor, and that won't get it all. If this is on a concrete slab and the cutback is thick or sticky, I'd suggest using 1/4" Hardibacker underlayment to give you a fresh surface for ceramic tile.
You could tile direct, but I'd advise this only after a successful test of adhesion between thinset mortar and the existing conditions. As it happens, I'm doing this very thing right now, but the cutback was very old, dry, and did not leave a thick coating. Also, the floor wasn't finished slick, so I'm getting a good grab for the thinset.