Remove now unneeded #undef in __cxa_guard.cpp
Before aosp/1086558, this was needed, since we would have
_USING_LIBCXX defined, yet <atomic> was not available, so we'd run
into compiler issues. After that change, we're using
__has_include(<atomic>) so this is no longer an issue.
Test: build
Change-Id: Ia81d343000e7d90f07422120fc1e1eb5becb01aa
diff --git a/libc/bionic/__cxa_guard.cpp b/libc/bionic/__cxa_guard.cpp
index 30b5f41..e2e7477 100644
--- a/libc/bionic/__cxa_guard.cpp
+++ b/libc/bionic/__cxa_guard.cpp
@@ -16,9 +16,7 @@
#include <endian.h>
#include <limits.h>
-#undef _USING_LIBCXX // Prevent using of <atomic>.
#include <stdatomic.h>
-
#include <stddef.h>
#include "private/bionic_futex.h"