[libc++][AIX] Remove LIBCXX-AIX-FIXME feature from test

This test was originally unsupported for `LIBCXX-AIX-FIXME` feature because we lacked `-fvisibility=hidden` support. AIX now has visibility support and the test passes in debug mode.

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D149255