nptl: Remove unnecessary forwarding of pthread_cond_clockwait from libc

In afe4de7d, I added forwarding functions
from libc to libpthread for __pthread_cond_clockwait and
pthread_cond_clockwait to mirror those for pthread_cond_timedwait. These
are unnecessary[1], since these functions aren't (yet) being called from
within libc itself. Let's remove them.

      * nptl/forward.c: Remove unnecessary __pthread_cond_clockwait and
	pthread_cond_clockwait forwarding functions.  There are no internal
	users, so it is unnecessary to expose these functions in libc.so.
	* sysdeps/nptl/pthread-functions.h (pthread_functions): Remove
	unnecessary ptr___pthread_cond_clockwait member.
	* nptl/nptl-init.c (pthread_functions): Remove assignment of
	removed member.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

[1] https://sourceware.org/ml/libc-alpha/2017-10/msg00082.html