Remove fallback support from eth1 service (#3594)

## Issue Addressed

N/A

## Proposed Changes

With https://github.com/sigp/lighthouse/pull/3214 we made it such that you can either have 1 auth endpoint or multiple non auth endpoints. Now that we are post merge on all networks (testnets and mainnet), we cannot progress a chain without a dedicated auth execution layer connection so there is no point in having a non-auth eth1-endpoint for syncing deposit cache. 

This code removes all fallback related code in the eth1 service. We still keep the single non-auth endpoint since it's useful for testing.

## Additional Info

This removes all eth1 fallback related metrics that were relevant for the monitoring service, so we might need to change the api upstream.

common/monitoring_api/src/gather.rs

@@ -43,6 +43,16 @@ impl JsonMetric {
             }
         }
     }
+
+    /// Return a default json value given given the metric type.
+    fn get_typed_value_default(&self) -> serde_json::Value {
+        match self.ty {
+            JsonType::Integer => json!(0),
+            JsonType::Boolean => {
+                json!(false)
+            }
+        }
+    }
 }
 
 /// The required metrics for the beacon and validator processes.
@@ -155,6 +165,16 @@ pub fn gather_metrics(metrics_map: &HashMap<String, JsonMetric>) -> Option<serde
             let _ = res.insert(metric.json_output_key.to_string(), value);
         };
     }
+    // Insert default metrics for all monitoring service metrics that do not
+    // exist as lighthouse metrics.
+    for json_metric in metrics_map.values() {
+        if !res.contains_key(json_metric.json_output_key) {
+            let _ = res.insert(
+                json_metric.json_output_key.to_string(),
+                json_metric.get_typed_value_default(),
+            );
+        }
+    }
     Some(serde_json::Value::Object(res))
 }