This helps with broadcasting txs from Chronik, which only has a `const NodeContext &` and no `const Config &`. Broadcasting from Chronik becomes more important once SLP is added.
Currently `BroadcastTransaction` takes a mutable `NodeContext &`, however, the mutability is not required by any of the functions called on it. It also takes a `const Config &` param, but that is unused, so we remove it.
Making it `const` is still thread safe as BroadcastTransaction takes a `LOCK(cs_main)`, and `RelayTransaction` (which isn't covered by that lock) has an internal per-node lock (see `CNode::PushTxInventory` and `PeerManagerImpl::RelayTransaction`).