Revert "Switch to the generated libcxx headers."

This reverts commit 280cbff037d6ebd4473dbcf138139fd7bb8db267.

Reason for revert:
This broke Cider G
https://chromium-review.googlesource.com/c/chromium/src/+/6768520/comments/13bb2d1d_3e90a250

Original change's description:
> Switch to the generated libcxx headers.
>
> This should prevent future issues upon libcxx roll.
>
> Fixed: 432582032
> Change-Id: I6a6a69649db0bca7576a421b2e7ba71c318ac537
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6768540
> Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
> Reviewed-by: Junji Watanabe <jwata@google.com>
> Commit-Queue: Matt Stark <msta@google.com>
> Cr-Commit-Position: refs/heads/main@{#1494581}

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Id5436163313cf188ce96f265b112e9fa15a30b9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6805052
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1494658}