Drop the `no-std` feature from `lightning-rapid-gossip-sync`

Drop the `no-std` feature from `lightning-rapid-gossip-sync`

Now that we don't have to have everything in our entire ecosystem
use the same `std`/`no-std` feature combinations we should start by
untangling our own features a bit.

This takes another step by removing the `no-std` feature entirely
from the `lightning-rapid-gossip-sync` crate and removing all
feature implications on dependencies from the remaining `std`
feature.