Cleanup light client leftovers (#11865)

* Remove --light cli option

* Cleanup light client leftovers

* Remove commented-out code and clean-up more light client leftovers

* Fix formatting with `cargo +nightly fmt`

* Remove FIXME regarding db directory structure

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/client/rpc-api/src/system/helpers.rs

@@ -76,8 +76,6 @@ pub struct PeerInfo<Hash, Number> {
 pub enum NodeRole {
 	/// The node is a full node
 	Full,
-	/// The node is a light client
-	LightClient,
 	/// The node is an authority
 	Authority,
 }