I don't recall what year the clutch interlock became standard equipment, but when it did there was also a recall to retrofit it to older models. When buying a used XJ, it might or might not have it, depending on whether or not the previous ownerr had the recall performed.
I would think that anything installed after the fact should also be able to be removed or bypassed fairly easily, but I don't have specifics on how the recall "feature" was wired. Just for laughs, try looking for a plunger switch mounted to the clutch pedal linkage under the dash.