Netd: remove redundant deletion for IPv6 mark rule.

When try to remove fwmark rules, netd will try to delete IPv6
fwmark rule multi times. The second time IP tools will return
254 as no corresponding rule existing. Netd will treat this
return value as failure and no further action can be performed.
This will cause VPN cannot be disconnected.

Change-Id: I00354cb90f4758e9d35b81e51e577db28a3da75c
CRs-fixed: 711085
netd: Add support for ipv6 tethering when using dun.

Change-Id: I24e4bf2cc1b10ee4f3196c87da392fc301b2a18a