VPN: do not use gateway in routing rules.

The first release of this API only supports TUN interface, and
so far there is no plan for TAP. TUN works on IP frames which
makes gateway nearly useless. The only value to have a gateway
is that kernel may use it to select the source address. However
currently it does not work in IPv6, and even in IPv4 applications
can bind to any local address on the device. Remove it to keep
the API clear, and it can be added back at any time if needed.

Change-Id: If9086d20d885a3a7f7ab5f07afbcfeecb677bbea