Revert "ndisc: Also set link hoplimit"

Revert "ndisc: Also set link hoplimit"

This reverts commit 0ba8374bb8a7032ca13ba98d10e8d7c3378ca9c6.

- link->network->ipv6_hop_limit must not be updated, as it may be used
  by multiple interfaces,
- We may receive multiple RA from multiple routers with different hop
  limit, and we should not make the default hop limit flicking,
- First of all, there is no reason that we need to update the default hop
  limit,
- The original issue should be caused by the hash and compare functions
  for Route object.