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

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

NOKEYCHECK=True
GitOrigin-RevId: a7adf7b9e9ffa7de9d90152ecf6430b800d2ca11
1 file changed