nptl: Remove unused internal futex functions

The __futex_abstimed_wait usage was remove with 3102e28b and the
__futex_abstimed_wait_cancelable by 323592fd and b8d3e8fb.
The futex_lock_pi can be replaced by a futex_lock_pi64.

Checked on x86_64-linux-gnu and i686-linux-gnu.

Reviewed-by: Lukasz Majewski <lukma@denx.de>