stop defining _LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS when building libc++abi

This was needed because libc++'s <exception> header omits
set_unexpected() (which libcxxabi defines) if that define isn't
set -- but <exception> also doesn't omit it if _LIBCPP_BUILDING_LIBRARY
is defined, which it is after
https://chromium-review.googlesource.com/c/chromium/src/+/3469341

No behavior change.

Bug: 1298070
Change-Id: I6b289656dc9bbc3a5428ac288bda54755bcf221d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3472583
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Arthur Eubanks <aeubanks@google.com>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/main@{#972605}
NOKEYCHECK=True
GitOrigin-RevId: 6e3c300d79ea1d1c57fddbf7414c0bb8624c35bc
diff --git a/third_party/libc++abi/BUILD.gn b/third_party/libc++abi/BUILD.gn
index 91839cc..40f1285 100644
--- a/third_party/libc++abi/BUILD.gn
+++ b/third_party/libc++abi/BUILD.gn
@@ -59,7 +59,6 @@
 
   defines = [
     "LIBCXXABI_SILENT_TERMINATE",
-    "_LIBCPP_ENABLE_CXX17_REMOVED_UNEXPECTED_FUNCTIONS",
 
     # TODO(crbug.com/1298070) _LIBCPP_CONSTINIT is defined in a libc++ revision
     # more recent than the one currently used in Chromium.