Remove AnyVersion and WildcardVersion constructors

These constructors don't carry any additional semantics.
They were used in `cabal check` implementation, but now already parser
reports errors so we don't need these redundant constructors anymore.

MajorBoundVersion (^>=) has slightly different semantics:
we want be able to relax only `^>= x.y.z` upper bounds.
Therefore it's preserved.