nptl: Assume __ASSUME_FUTEX_CLOCK_REALTIME support

This patch assumes realtime clock support for nptl and thus removes
all the associated code.

For __pthread_mutex_timedlock the fallback usage for the case where
lll_futex_timed_wait_bitset it not set define is also removed. The
generic lowlevellock-futex.h always define it, so for NPTL code the
check always yield true.

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

	* nptl/nptl-init.c (__have_futex_clock_realtime,
	__have_futex_clock_realtime): Remove definition.
	(__pthread_initialize_minimal_internal): Remove FUTEX_CLOCK_REALTIME
	check test for !__ASSUME_FUTEX_CLOCK_REALTIME.
	* nptl/pthread_mutex_timedlock.c (__pthread_mutex_timedlock): Assume
	__ASSUME_FUTEX_CLOCK_REALTIME support.
	* sysdeps/unix/sysv/linux/i386/lowlevellock.S: Likewise.
	* sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: Likewise.
	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_FUTEX_CLOCK_REALTIME): Remove.
	* sysdeps/nptl/lowlevellock-futex.h (lll_futex_timed_wait_bitset):
	Adjust comment.