[Sema] NFC: Remove unused declaration

`deriveProtocolRequirement` is no longer a
member on `TypeChecker`, it's static.

Author: hamishknight

lib/Sema/DerivedConformances.cpp

@@ -280,7 +280,7 @@ void DerivedConformance::diagnoseIfSynthesisUnsupportedForDecl(
 ValueDecl *DerivedConformance::getDerivableRequirement(NominalTypeDecl *nominal,
                                                        ValueDecl *requirement) {
   // Note: whenever you update this function, also update
-  // TypeChecker::deriveProtocolRequirement.
+  // deriveProtocolRequirement.
   ASTContext &ctx = nominal->getASTContext();
   const auto name = requirement->getName();
 

lib/Sema/TypeChecker.h

@@ -826,21 +826,6 @@ void checkConformancesInContext(IterableDeclContext *idc);
 /// Check that the type of the given property conforms to NSCopying.
 ProtocolConformanceRef checkConformanceToNSCopying(VarDecl *var);
 
-/// Derive an implicit declaration to satisfy a requirement of a derived
-/// protocol conformance.
-///
-/// \param DC           The declaration context where the conformance was
-///                     defined, either the type itself or an extension
-/// \param TypeDecl     The type for which the requirement is being derived.
-/// \param Requirement  The protocol requirement.
-///
-/// \returns nullptr if the derivation failed, or the derived declaration
-///          if it succeeded. If successful, the derived declaration is added
-///          to TypeDecl's body.
-ValueDecl *deriveProtocolRequirement(DeclContext *DC,
-                                     NominalTypeDecl *TypeDecl,
-                                     ValueDecl *Requirement);
-
 /// \name Name lookup
 ///
 /// Routines that perform name lookup.