nptl: Remove futex_supports_exact_relative_timeouts

nptl: Remove futex_supports_exact_relative_timeouts

The only implementation of futex_supports_exact_relative_timeouts always
returns true. Let's remove it and all its callers.

	* nptl/pthread_cond_wait.c: (__pthread_cond_clockwait): Remove code
	that is only useful if futex_supports_exact_relative_timeouts ()
	returns false.
	* nptl/pthread_condattr_setclock.c: (pthread_condattr_setclock):
	Likewise.
	* sysdeps/nptl/futex-internal.h: Remove comment about relative
	timeouts potentially being imprecise since it's no longer true.
	Remove declaration of futex_supports_exact_relative_timeouts.
	* sysdeps/unix/sysv/linux/futex-internal.h: Remove implementation
	of futex_supports_exact_relative_timeouts.

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