swift234

[Constraint system] Eliminate DependentTypeOpener. NFC

Eliminate the last client of DependentTypeOpener,
RequirementTypeOpener, which tracked the opened Self type when doing
witness/requirement matching and substituted in the known type
witnesses for that protocol. It had a bunch of dead logic hanging
around from the days where we used the constraint system to deduce
type witnesses. Now, a simple substitution suffices.

With its last client gone, remove DependentTypeOpener as well.