Define ss58 prefix inside the runtime (#7810)

* Add SS58Prefix type to the frame_system config trait

* Remove unused chain_id runtime interface

substrate/primitives/externalities/src/lib.rs

@@ -170,9 +170,6 @@ pub trait Externalities: ExtensionStore {
 		value: Option<Vec<u8>>,
 	);
 
-	/// Get the identity of the chain.
-	fn chain_id(&self) -> u64;
-
 	/// Get the trie root of the current storage map.
 	///
 	/// This will also update all child storage keys in the top-level storage map.