nptl: Remove private futex optimization [BZ #27304]

It is effectively used, unexcept for pthread_cond_destroy, where we do
not want it; see bug 27304.  The internal locks do not support a
process-shared mode.

This fixes commit dc6cfdc9 ("nptl:
Move pthread_cond_destroy implementation into libc").

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
(cherry picked from commit c4ad8322)