libc++: Stop defining _LIBCPP_CHAR_TRAITS_REMOVE_BASE_SPECIALIZATION

This no longer has an effect since
llvm/llvm-project#72694 which we rolled in in
https://chromium-review.googlesource.com/c/chromium/src/+/5261503
in early February.

No behavior change.

Change-Id: I20f32af0296f17a917c64a53f5e0f78a07359a39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5753085
Commit-Queue: Arthur Eubanks <[email protected]>
Auto-Submit: Nico Weber <[email protected]>
Reviewed-by: Arthur Eubanks <[email protected]>
Commit-Queue: Nico Weber <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1335791}
NOKEYCHECK=True
GitOrigin-RevId: 17bda910cb33fefeb8d4f0d96064ce72d5ac0c44

Author: nico

third_party/libc++/__config_site

@@ -84,7 +84,4 @@
 #define _LIBCPP_HAS_NO_INT128
 #endif
 
-// TODO(thakis): Remove this after LLVM 19's libc++ is rolled in.
-#define _LIBCPP_CHAR_TRAITS_REMOVE_BASE_SPECIALIZATION
-
 #endif // _LIBCPP_CONFIG_SITE