Remove genesis block hash from public API

Remove genesis block hash from public API

Forcing users to pass a genesis block hash has ended up being
error-prone largely due to byte-swapping questions for bindings
users. Further, our API is currently inconsistent - in
`ChannelManager` we take a `Bitcoin::Network` but in `NetworkGraph`
we take the genesis block hash.

Luckily `NetworkGraph` is the only remaining place where we require
users pass the genesis block hash, so swapping it for a `Network`
is a simple change.