[clang] remove unused frontend flag -fretain_subst_template_type_parm_type_ast_nodes

This is a folow-up to #132748, where we deferred the flag removal
in order to ease transition for external users.

Author: mizvekov

clang/include/clang/Basic/LangOptions.def

@@ -161,7 +161,6 @@ LANGOPT(Coroutines        , 1, 0, "C++20 coroutines")
 LANGOPT(CoroAlignedAllocation, 1, 0, "prefer Aligned Allocation according to P2014 Option 2")
 LANGOPT(DllExportInlines  , 1, 1, "dllexported classes dllexport inline methods")
 LANGOPT(ExperimentalLibrary, 1, 0, "enable unstable and experimental library features")
-LANGOPT(RetainSubstTemplateTypeParmTypeAstNodes, 1, 0, "retain SubstTemplateTypeParmType nodes in the AST's representation of alias template specializations")
 
 LANGOPT(PointerAuthIntrinsics, 1, 0, "pointer authentication intrinsics")
 LANGOPT(PointerAuthCalls  , 1, 0, "function pointer authentication")

clang/include/clang/Driver/Options.td

@@ -3508,12 +3508,6 @@ defm application_extension : BoolFOption<"application-extension",
   PosFlag<SetTrue, [], [ClangOption, CC1Option],
           "Restrict code to those available for App Extensions">,
   NegFlag<SetFalse>>;
-defm retain_subst_template_type_parm_type_ast_nodes : BoolFOption<"retain-subst-template-type-parm-type-ast-nodes",
-  LangOpts<"RetainSubstTemplateTypeParmTypeAstNodes">, DefaultFalse,
-  PosFlag<SetTrue, [], [CC1Option], "Enable">,
-  NegFlag<SetFalse, [], [], "Disable">,
-  BothFlags<[], [], " retain SubstTemplateTypeParmType nodes in the AST's representation"
-  " of alias template specializations">>;
 defm sized_deallocation : BoolFOption<"sized-deallocation",
   LangOpts<"SizedDeallocation">, Default<cpp14.KeyPath>,
   PosFlag<SetTrue, [], [], "Enable C++14 sized global deallocation functions">,