Cleanup ChannelId re-export

`ChannelId` was weirdly listed in the re-export section of the docs and
reachable via multiple paths. Here we opt to make the `channel_id`
module private and leave only the `ChannelId` struct itself exposed.

Author: tnull

lightning/src/ln/mod.rs

@@ -13,14 +13,14 @@
 #[macro_use]
 pub mod functional_test_utils;
 
-pub mod channel_id;
 pub mod channelmanager;
 pub mod inbound_payment;
 pub mod msgs;
 pub mod peer_handler;
 pub mod chan_utils;
 pub mod features;
 pub mod script;
+mod channel_id;
 
 #[cfg(fuzzing)]
 pub mod peer_channel_encryptor;
@@ -33,7 +33,7 @@ pub mod channel;
 pub(crate) mod channel;
 
 // Re-export ChannelId
-pub use self::channel_id::ChannelId;
+pub use channel_id::ChannelId;
 
 pub(crate) mod onion_utils;
 mod outbound_payment;