Remove __ASSUME_SOCK_CLOEXEC / SOCK_CLOEXEC conditionals in Linux-specific code.

Remove __ASSUME_SOCK_CLOEXEC / SOCK_CLOEXEC conditionals in Linux-specific code.

This patch removes conditionals on __ASSUME_SOCK_CLOEXEC, and on
SOCK_CLOEXEC being defined, in Linux-specific code, now that all
supported Linux kernel versions can be assumed to have this
functionality.  (The macro is also used in OS-independent code and is
not defined for Hurd.)

Tested x86_64 that the disassembly of installed shared libraries is
unchanged by this patch.

	* nptl/sysdeps/unix/sysv/linux/mq_notify.c: Do not include
	<kernel-features.h>.
	(init_mq_netlink): Remove conditional have_sock_cloexec
	definitions.  Remove code conditional on have_sock_cloexec < 0.
	(init_mq_netlink) [!SOCK_CLOEXEC]: Remove conditional code.
	(init_mq_netlink) [!__ASSUME_SOCK_CLOEXEC]: Likewise.
	* sysdeps/unix/sysv/linux/opensock.c: Do not include
	<kernel-features.h>.
	(__opensock) [SOCK_CLOEXEC]: Make code unconditional.
	(__opensock) [!__ASSUME_SOCK_CLOEXEC]: Remove conditional code.