Remove __ASSUME_SOCKETCALL.

The __ASSUME_SOCKETCALL macro in kernel-features.h is no longer used
for anything.  (It used to be used in defining other macros related to
accept4 / recvmmsg / sendmmsg availability, but the code in that area
was simplified once we could assume a kernel with those features,
whether through a syscall or through socketcall, so allowing those
functions to be handled much like other socket operations, without
requring __ASSUME_SOCKETCALL.)  This patch removes that unused macro.

(Note: once we can assume a Linux 4.4 or later kernel, much of the
support for using socketcall at all can be removed from glibc,
although a few functions may need that support in glibc for longer.)

Tested with build-many-glibcs.py.

	* sysdeps/unix/sysv/linux/kernel-features.h: Remove comment about
	__ASSUME_SOCKETCALL.
	* sysdeps/unix/sysv/linux/i386/kernel-features.h
	(__ASSUME_SOCKETCALL): Remove.
	* sysdeps/unix/sysv/linux/m68k/kernel-features.h
	(__ASSUME_SOCKETCALL): Likewise.
	* sysdeps/unix/sysv/linux/microblaze/kernel-features.h
	(__ASSUME_SOCKETCALL): Likewise.
	* sysdeps/unix/sysv/linux/powerpc/kernel-features.h
	(__ASSUME_SOCKETCALL): Likewise.
	* sysdeps/unix/sysv/linux/s390/kernel-features.h
	(__ASSUME_SOCKETCALL): Likewise.
	* sysdeps/unix/sysv/linux/sh/kernel-features.h
	(__ASSUME_SOCKETCALL): Likewise.
	* sysdeps/unix/sysv/linux/sparc/kernel-features.h
	(__ASSUME_SOCKETCALL): Likewise.

sysdeps/unix/sysv/linux/sparc/kernel-features.h

@@ -17,9 +17,6 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
-/* SPARC uses socketcall.  */
-#define __ASSUME_SOCKETCALL		1
-
 #include_next <kernel-features.h>
 
 /* 32-bit SPARC kernels do not support