[libcxx][AIX] Remove `LIBCXX-AIX-FIXME` in isalwayslockfree check after 86478c7ad8

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

Reviewed By: ldionne, daltenty, jsji, #libc

Differential Revision: https://reviews.llvm.org/D121081

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

@@ -8,9 +8,6 @@
 //
 // UNSUPPORTED: c++03, c++11, c++14
 
-// Fails for 32-bit builds.
-// UNSUPPORTED: LIBCXX-AIX-FIXME
-
 // <atomic>
 
 // static constexpr bool is_always_lock_free;