Remove remaining _HAVE_STRING_ARCH_* definitions (BZ #18858)

Since the removal of bits/string.h, _HAVE_STRING_ARCH_* are no
longer used.  This patch removes the unused macros from i686
and x86_64 sysdeps folder.

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

	* sysdeps/i386/i686/multiarch/strncpy.c (_HAVE_STRING_ARCH_strncpy):
	Remove define.
	* sysdeps/x86_64/multiarch/stpcpy.c (_HAVE_STRING_ARCH_stpcpy):
	Likewise.
	* sysdeps/x86_64/multiarch/strcspn.c (_HAVE_STRING_ARCH_strcspn):
	Likewise.
	* sysdeps/x86_64/multiarch/strncat.c (_HAVE_STRING_ARCH_strncat):
	Likewise.
	* sysdeps/x86_64/multiarch/strncpy.c (_HAVE_STRING_ARCH_strncpy):
	Likewise.
	* sysdeps/x86_64/multiarch/strpbrk.c (_HAVE_STRING_ARCH_strpbrk):
	Likewise.
	* sysdeps/x86_64/multiarch/strspn.c (_HAVE_STRING_ARCH_strspn):
	Likewise.