[libc++][NFC] Remove #endif comments for really small conditionals on _LIBCPP_HAS_NO_UNICODE_CHARS

We generally don't put a comment on the #endif when the #if block is so small
that it's unambiguous what the #endif refers to.

NOKEYCHECK=True
GitOrigin-RevId: 71752e00081cb4bc55d60c186cce1a18ed12e405
diff --git a/include/string b/include/string
index 8696bdf..4ca0e86 100644
--- a/include/string
+++ b/include/string
@@ -675,7 +675,7 @@
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
 typedef basic_string<char16_t> u16string;
 typedef basic_string<char32_t> u32string;
-#endif // _LIBCPP_HAS_NO_UNICODE_CHARS
+#endif
 
 template<class _CharT, class _Traits, class _Allocator>
 class