nptl: Remove __ASSUME_PRIVATE_FUTEX

Since __ASSUME_PRIVATE_FUTEX is always defined, this patch removes the
!__ASSUME_PRIVATE_FUTEX paths.

Tested with build-many-glibcs.py.

	* nptl/allocatestack.c (allocate_stack): Remove the
	!__ASSUME_PRIVATE_FUTEX paths.
	* nptl/descr.h (header): Remove the !__ASSUME_PRIVATE_FUTEX path.
	* nptl/nptl-init.c (__pthread_initialize_minimal_internal):
	Likewise.
	* sysdeps/i386/nptl/tcb-offsets.sym (PRIVATE_FUTEX): Removed.
	* sysdeps/powerpc/nptl/tcb-offsets.sym (PRIVATE_FUTEX): Likewise.
	* sysdeps/sh/nptl/tcb-offsets.sym (PRIVATE_FUTEX): Likewise.
	* sysdeps/x86_64/nptl/tcb-offsets.sym (PRIVATE_FUTEX): Likewise.
	* sysdeps/i386/nptl/tls.h: (tcbhead_t): Remve the
	!__ASSUME_PRIVATE_FUTEX path.
	* sysdeps/s390/nptl/tls.h (tcbhead_t): Likewise.
	* sysdeps/sparc/nptl/tls.h (tcbhead_t): Likewise.
	* sysdeps/x86_64/nptl/tls.h (tcbhead_t): Likewise.
	* sysdeps/unix/sysv/linux/i386/lowlevellock.S: Remove the
	!__ASSUME_PRIVATE_FUTEX macros.
	* sysdeps/unix/sysv/linux/lowlevellock-futex.h: Likewise.
	* sysdeps/unix/sysv/linux/x86_64/cancellation.S: Likewise.
	* sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: Likewise.
	* sysdeps/unix/sysv/linux/kernel-features.h
	(__ASSUME_PRIVATE_FUTEX): Removed.