Remove unused fields from struct in6_ifextra

[Klemens Nanni]

This is all under _KERNEL:
- rs_lhcookie was added in 2014 110585f259f4974284e531f0a1e121b001a580dc
  "Move sending of router solicitations to the kernel; [...]"
  but never used
- nprefixes and ndefrouters became obsolete with 2017
  4a2f474d14c160dc7829cce0149ead09d473ece9
  "Remove sending of router solicitations and processing of router
   advertisements from the kernel. [...]"

OK?

Index: netinet6/in6_var.h
===================================================================
RCS file: /cvs/src/sys/netinet6/in6_var.h,v
retrieving revision 1.74
diff -u -p -r1.74 in6_var.h
--- netinet6/in6_var.h  2 Jan 2022 22:36:04 -0000       1.74
+++ netinet6/in6_var.h  12 Nov 2022 03:00:53 -0000
@@ -90,9 +90,6 @@ struct in6_addrlifetime {
 struct nd_ifinfo;
 struct in6_ifextra {
        struct nd_ifinfo *nd_ifinfo;
-       void *rs_lhcookie;
-       int nprefixes;
-       int ndefrouters;
 };
 
 struct in6_ifaddr {
Index: netinet6/in6.c
===================================================================
RCS file: /cvs/src/sys/netinet6/in6.c,v
retrieving revision 1.249
diff -u -p -r1.249 in6.c
--- netinet6/in6.c      8 Sep 2022 10:22:06 -0000       1.249
+++ netinet6/in6.c      12 Nov 2022 03:00:37 -0000
@@ -1602,8 +1602,6 @@ in6_domifattach(struct ifnet *ifp)
        ext = malloc(sizeof(*ext), M_IFADDR, M_WAITOK | M_ZERO);
 
        ext->nd_ifinfo = nd6_ifattach(ifp);
-       ext->nprefixes = 0;
-       ext->ndefrouters = 0;
        return ext;
 }