[NFC] ProtocolDecl: Remove some dead friend declarations

Author: AnthonyLatsis

include/swift/AST/Decl.h

@@ -4786,8 +4786,6 @@ class ProtocolDecl final : public NominalTypeDecl {
   friend class TypeAliasRequirementsRequest;
   friend class ProtocolDependenciesRequest;
   friend class RequirementSignatureRequest;
-  friend class RequirementSignatureRequestRQM;
-  friend class RequirementSignatureRequestGSB;
   friend class ProtocolRequiresClassRequest;
   friend class ExistentialConformsToSelfRequest;
   friend class HasSelfOrAssociatedTypeRequirementsRequest;