Revert listening on IPv6 by default

[tomaka]

It leads to the following errors:

2020-04-17 16:40:25 Libp2p listener ListenerId(1) closed
2020-04-17 16:40:25 Libp2p => ListenerClosed: Err(Custom { kind: Other, error: Custom { kind: Other, error: Other(A(A(A(B(Underlying(B(Os { code: 98, kind: AddrInUse, message: "Address already in use" }))))))) } })

...

2020-04-17 16:43:13 Error on libp2p listener ListenerId(2): ::ffff:127.0.0.1 does not match any listen address
2020-04-17 16:43:13 Error on libp2p listener ListenerId(2): ::ffff:10.128.0.9 does not match any listen address

Let's revert this for the release of Polkadot 0.7.30 and open an issue instead.