Set libc++ availability minimum header version to a large value

This removes a temporary workaround introduced in
https://chromium-review.googlesource.com/c/chromium/src/+/7495232
since we recently rolled in the fix [1] for the problem it worked
around.

1: http://llvm.org/PR193183

Bug: 503250165
Change-Id: I56c36a41955437a9f63ffb517bbf72c62d60d3a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7786170
Commit-Queue: Hans Wennborg <hans@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1621062}
NOKEYCHECK=True
GitOrigin-RevId: 456cbe0e8ef8f4bb2552327ba38019177e7e5d4a