Fix/vpn server close

[ersonp]

Did you run make format && make check?
yes

Fixes #1153

Changes:

• Reverted network changes on vpn-server close

How to test this PR:

1. Start visor sudo ./skywire-visor -c skywire-config.json
2. Check the outputs of commands
   sysctl net.ipv4.ip_forward
sysctl net.ipv6.conf.all.forwarding
iptables -L | grep "Chain FORWARD" | tr -d '()' | awk '{print $4}'
3. Start vpn-server
4. Check the outputs of commands
   sysctl net.ipv4.ip_forward
sysctl net.ipv6.conf.all.forwarding
iptables -L | grep "Chain FORWARD" | tr -d '()' | awk '{print $4}'
5. Stop vpn-server
6. Check the outputs of commands
   sysctl net.ipv4.ip_forward
sysctl net.ipv6.conf.all.forwarding
iptables -L | grep "Chain FORWARD" | tr -d '()' | awk '{print $4}'
7. Everything should be reverted