[clang] Remove unused declaration IgnoreLinkageSpecDecls

The declaration and its corresponding function definition were removed
once by:

  commit 0209390
  Author: Nathan Sidwell <nathan@acm.org>
  Date:   Mon Feb 14 10:19:04 2022 -0800

However, the declaration was added back without a corresponding
function definition a few days later by:

  commit 18ead23
  Author: Peter Collingbourne <peter@pcc.me.uk>
  Date:   Thu Feb 17 11:23:33 2022 -0800

This is most likely a rebasing error.

clang/lib/AST/ItaniumMangle.cpp

@@ -208,7 +208,6 @@ class ItaniumMangleContextImpl : public ItaniumMangleContext {
   }
 
   bool isInternalLinkageDecl(const NamedDecl *ND);
-  const DeclContext *IgnoreLinkageSpecDecls(const DeclContext *DC);
 
   /// @}
 };
@@ -396,7 +395,6 @@ class CXXNameMangler {
   bool isStdNamespace(const DeclContext *DC);
 
   const RecordDecl *GetLocalClassDecl(const Decl *D);
-  const DeclContext *IgnoreLinkageSpecDecls(const DeclContext *DC);
   bool isSpecializedAs(QualType S, llvm::StringRef Name, QualType A);
   bool isStdCharSpecialization(const ClassTemplateSpecializationDecl *SD,
                                llvm::StringRef Name, bool HasAllocator);