Remove unused relays/headers (#1216)

* Decouple `relays/client-substrate` from `headers_relay`
* Remove `blocks_in_state` from `SyncLoopMetrics`
    This metric was only relevant for PoA <> Substrate bridge.
* Move `sync_loop_metrics.rs` to `relays/finality`
* Remove unused `SyncLoopMetrics::update()`
* Hook up SyncLoopMetrics to finality_loop
* Delete now unused `relays/headers`

Signed-off-by: acatangiu <adrian@parity.io>

bridges/relays/finality/src/finality_loop.rs

@@ -19,12 +19,13 @@
 //! is the mandatory headers, which we always submit to the target node. For such headers, we
 //! assume that the persistent proof either exists, or will eventually become available.
 
-use crate::{FinalityProof, FinalitySyncPipeline, SourceHeader};
+use crate::{
+	sync_loop_metrics::SyncLoopMetrics, FinalityProof, FinalitySyncPipeline, SourceHeader,
+};
 
 use async_trait::async_trait;
 use backoff::backoff::Backoff;
 use futures::{select, Future, FutureExt, Stream, StreamExt};
-use headers_relay::sync_loop_metrics::SyncLoopMetrics;
 use num_traits::{One, Saturating};
 use relay_utils::{
 	metrics::{GlobalMetrics, MetricsParams},