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.

beacon_node/src/lib.rs

@@ -117,7 +117,7 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
             info!(
                 log,
                 "Block production enabled";
-                "endpoints" => format!("{:?}", &client_config.eth1.endpoints),
+                "endpoint" => format!("{:?}", &client_config.eth1.endpoint),
                 "method" => "json rpc via http"
             );
             builder