Depends on age of vehicle. I beleive up to K, if the car was fitted with one, it MUST remain, but you can wrangle it. After K, it must have it. Dates may be wrong but thats the gist.
Audi is an H reg, never batted an eyelid at the fact it has been de-catted. even though the rear window proclaimes 'Shon Mit Katalyzer Technic' (Fitted with catalyser technology)