Refactor ParamBounds away completely

[nikomatsakis]

Once PR #20159 lands, there will be only one (I believe) major remaining use of ParamBounds, which is that it is used to store the list of supertraits. We should remove ParamBounds completely and just use the generic predicate lists we've been working towards.

cc @jroesch

[jroesch]

I looked at this a little bit this morning and it will be fun to finally remove this.

[steveklabnik]

Triage: there's no more Parambounds, but there is a TyParamBounds. Is that the same thing?

[nikomatsakis]

I think this is done.