Remove PS_STRINGS fallback from setproctitle

In r103767 the kern.ps_strings sysctl was added as the preferred way to
locate the ps_strings struct and is available in any FreeBSD release
supported within the last decade.

Reviewed by:	kib

lib/libc/gen/setproctitle.c

@@ -134,7 +134,7 @@ setproctitle(const char *fmt, ...)
 		len = sizeof(ul_ps_strings);
 		if (sysctlbyname("kern.ps_strings", &ul_ps_strings, &len, NULL,
 		    0) == -1)
-			ul_ps_strings = PS_STRINGS;
+			return;
 		ps_strings = (struct ps_strings *)ul_ps_strings;
 	}