[AST] [NFC] Remove unused field from SubstTemplateTypeParmPackType

The field has been moved to the base class, but I forgot to actually
delete it in the derived class.

Author: ilya-biryukov

clang/include/clang/AST/TypeBase.h

@@ -7082,10 +7082,6 @@ class SubstBuiltinTemplatePackType : public SubstPackType {
 class SubstTemplateTypeParmPackType : public SubstPackType {
   friend class ASTContext;
 
-  /// A pointer to the set of template arguments that this
-  /// parameter pack is instantiated with.
-  const TemplateArgument *Arguments;
-
   llvm::PointerIntPair<Decl *, 1, bool> AssociatedDeclAndFinal;
 
   SubstTemplateTypeParmPackType(QualType Canon, Decl *AssociatedDecl,