[RFCLAT#5] remove the unnecessary tun fd, socket handler and socket opt cleanup

- Closing tun fd is not necessary because any open fds are closed
  when a process exits
- Closing socket fds are not necessary because any open fds are
  closed when a process exits
- Applying IPV6_LEAVE_ANYCAST is not necessary because the setting is
  stored with the socket and removed when it is closed
  See addrconf_ifdown, ipv6_ac_destroy_dev

Bug: 212345928
Test: atest clatd_test and manual test
1. Connect to ipv6-only wifi.
2. Try IPv4 traffic.
   $ ping 8.8.8.8

Change-Id: I8002c1adef208f5dcd6665fecee0df6236159f88
1 file changed