Remove the builtin_headers_in_system_modules feature (#75262)

__has_feature(builtin_headers_in_system_modules) was added in
https://reviews.llvm.org/D159483 to be used in the stdarg/stddef
implementation headers. It ended up being unnecessary, but I forgot to
remove the feature definition.

Author: ian-twilightcoder

clang/include/clang/Basic/Features.def

@@ -282,7 +282,6 @@ EXTENSION(matrix_types_scalar_division, true)
 EXTENSION(cxx_attributes_on_using_declarations, LangOpts.CPlusPlus11)
 EXTENSION(datasizeof, LangOpts.CPlusPlus)
 
-FEATURE(builtin_headers_in_system_modules, LangOpts.BuiltinHeadersInSystemModules)
 FEATURE(cxx_abi_relative_vtable, LangOpts.CPlusPlus && LangOpts.RelativeCXXABIVTables)
 
 // CUDA/HIP Features

clang/test/Driver/darwin-builtin-modules.c

@@ -9,19 +9,3 @@
 // RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos98.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
 // RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos99.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
 // CHECK_FUTURE-NOT: -fbuiltin-headers-in-system-modules
-
-
-// Check that builtin_headers_in_system_modules is only set if -fbuiltin-headers-in-system-modules and -fmodules are both set.
-
-// RUN: %clang -isysroot %S/Inputs/iPhoneOS13.0.sdk -target arm64-apple-ios13.0 -fsyntax-only %s -Xclang -verify=no-feature
-// RUN: %clang -isysroot %S/Inputs/iPhoneOS13.0.sdk -target arm64-apple-ios13.0 -fsyntax-only %s -fmodules -Xclang -verify=yes-feature
-// RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos99.0 -fsyntax-only %s -Xclang -verify=no-feature
-// RUN: %clang -isysroot %S/Inputs/MacOSX99.0.sdk -target x86_64-apple-macos99.0 -fsyntax-only %s -fmodules -Xclang -verify=no-feature
-
-#if __has_feature(builtin_headers_in_system_modules)
-#error "has builtin_headers_in_system_modules"
-// yes-feature-error@-1 {{}}
-#else
-#error "no builtin_headers_in_system_modules"
-// no-feature-error@-1 {{}}
-#endif