I don't really know anything about this particular stamp, but a quick check of auction sites suggests the stamp can be had for less than $1, so the secondary market value is relatively minimal.
I think the Scott reference to "withdrawn from sale the week of release" may pertain to its first day of issue at World Stamp Expo '89. The details are recited in the link below and the few (approx. 80) FDCs that were made do command high prices. However, the commentary goes on to suggest that only "for a short time" the PO suspended their sale:
http://www.raylcoughlin.com/store/i...39_5531_5532To answer your specific question as to whether or not you should soak off that stamp, I would err on the side of leaving it on cover. The postmark is clear (at least the date is) and the cover is in good shape and may command more value in the future in that state than the stamp would alone. Of course, the final decision is one that only you can make.