Two fixes to handling of abstract types with cap bounds (#22838)

1. Strip implied cap.rd sets from Capability types in extends and bounds

    I.e. We do not add an implicit cap.rd in

       class C extends Capability
       type T <: C

2. Don't map CapSet^ to CapSet^{fresh}. Assume `^` is root in some
comparisons with CapSet.

Author: odersky