build: Remove no-longer-needed _LIBCPP_CONSTINIT define

We had to define this due to an earlier libc++ roll removing the define
while our libc++abi roller was stuck.

Now that we finally rolled libc++abi, this is no longer needed.

No behavior change.

Bug: 469458284,503250165
Change-Id: I1f9363782398efa98d941c7f059df5dd677f3bdc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8182141
Reviewed-by: Arthur Eubanks <aeubanks@google.com>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1672810}
NOKEYCHECK=True
GitOrigin-RevId: efad31f49b040c172bf4d107aab78f9bb53ab075