futex: Remove not used futex_reltimed_wait{_cancelable}

futex: Remove not used futex_reltimed_wait{_cancelable}

After gai_suspend and aio_suspend conversion to support 64 bit time and
hence rewriting the code to use only absolute variants of futex wait
functions (i.e. __futex_abstimed_wait64 and __futex_abstimed_wait_cancelable64)
futex_reltimed_wait{_cancelable} are not needed anymore and can be removed.

Build tests:
./src/scripts/build-many-glibcs.py glibcs

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>