[Conformance checker] Eliminate redundant recording of missing witnesses.

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -3800,8 +3800,7 @@ ResolveWitnessResult ConformanceChecker::resolveWitnessViaDefault(
     recordOptionalWitness(requirement);
     return ResolveWitnessResult::Success;
   }
-  // Save the missing requirement for later diagnosis.
-  GlobalMissingWitnesses.insert({requirement, {}});
+
   return ResolveWitnessResult::ExplicitFailed;
 }