Remove unused fields from struct in6_ifextra

Remove unused fields from struct in6_ifextra

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 mpi