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/service/src/metrics.rs

@@ -160,7 +160,7 @@ impl MetricsService {
 	) -> Result<Self, PrometheusError> {
 		let role_bits = match config.role {
 			Role::Full => 1u64,
-			Role::Light => 2u64,
+			// 2u64 used to represent light client role
 			Role::Authority { .. } => 4u64,
 		};