Remove "silent superclass parameters"
We introduced silent superclass parameters as a way to avoid superclass loops, but we now solve that problem a different way ("derived" superclass constraints carry no evidence). So they aren't needed any more. Apart from being a needless complication, they broke DoCon. Admittedly in a very obscure way, but still the result is hard to explain. To see the details see Trac #5051, with test case typecheck/should_compile/T5051. (The test is nice and small!)