Revert "buildtools: Copy lib++ headers for Clang modules build"

This reverts commit 7a8b870b60.

Reason for revert: It breaks https://ci.chromium.org/ui/p/chrome/builders/ci/android-arm/44908/overview

Original change's description:
> buildtools: Copy lib++ headers for Clang modules build
>
> This is to have libc++ headers and __assertion_handler in the same
> directory for Clang modules build.
>
> Bug: 408128305
> Change-Id: Iad590d053e7fcc7a3414ad5b6e1249126eca5b9b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6440444
> Reviewed-by: Junji Watanabe <jwata@google.com>
> Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1444568}

Bug: 408128305
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Iaeb4e9f3c9c730dcb3ad95df0ff64497e40c9a56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6440532
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Junji Watanabe <jwata@google.com>
Cr-Commit-Position: refs/heads/main@{#1444586}

buildtools/third_party/libc++/BUILD.gn

@@ -6,7 +6,6 @@ import("//build/config/c++/c++.gni")
 import("//build/config/clang/clang.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/toolchain.gni")
-import("//buildtools/third_party/libc++/libcxx_headers.gni")
 
 # Used by libc++ and libc++abi.
 # See //build/config/c++:runtime_library for the config used by users of libc++.
@@ -125,32 +124,14 @@ configs_to_remove = [
   "//build/config/coverage:default_coverage",
 ]
 
-# We need to have libc++ headers and __assertion_handler in the same directory
-# for clang modules build.
-copy("copy_libcxx_headers") {
-  sources = libcxx_headers
-  outputs = [ "{{source_gen_dir}}/{{source_file_part}}" ]
-}
-
-copy("copy_assertion_handler") {
-  sources = [ "__assertion_handler" ]
-  outputs =
-      [ "${root_gen_dir}/third_party/libc++/src/include/__assertion_handler" ]
-}
-
 template("libcxx_modules") {
   # This is a template to build libc++'s module file.
   source_set(target_name) {
     use_libcxx_modules = false
     if (use_explicit_libcxx_modules) {
-      sources =
-          [ "${root_gen_dir}/third_party/libc++/src/include/module.modulemap" ]
+      sources = [ "//third_party/libc++/src/include/module.modulemap" ]
     }
-
-    deps = [
-      ":copy_assertion_handler",
-      ":copy_libcxx_headers",
-    ]
+    deps = []
 
     configs -= configs_to_remove
     configs += configs_to_add