Disable incoming light-client connections for minimal relay node (#2202)

When running with `--relay-chain-rpc-url` we received multiple reports
of high traffic that disappears when `--in-peers-light 0` is set. Indeed
it does not make much sense for light clients to connect to the minimal
node since it is not running the block announce protocol and the
request/response protocol for light clients.

This is intended to alleviate the traffic issues for now.

closes #1896
probably related https://github.com/paritytech/cumulus/issues/2563