[libcxx][AIX] Remove `LIBCXX-AIX-FIXME` in isalwayslockfree check after 86478c7ad8a7

[lkail]

Summary

The workaround has been removed via https://reviews.llvm.org/D119931, checks passed on AIX-32.

[jsji]

LGTM. Thanks.

libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: c++03, c++11, c++14
 
-// Fails for 32-bit builds.
-// UNSUPPORTED: LIBCXX-AIX-FIXME
-
 // <atomic>
 
 // static constexpr bool is_always_lock_free;
 
 #include <atomic>
 #include <cassert>
 
 #include "test_macros.h"