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/cli.rs

@@ -372,9 +372,9 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .long("eth1-endpoints")
                 .value_name("HTTP-ENDPOINTS")
                 .conflicts_with("eth1-endpoint")
-                .help("One or more comma-delimited server endpoints for web3 connection. \
-                       If multiple endpoints are given the endpoints are used as fallback in the \
-                       given order. Also enables the --eth1 flag. \
+                .help("One http endpoint for a web3 connection to an execution node. \
+                       Note: This flag is now only useful for testing, use `--execution-endpoint` \
+                       flag to connect to an execution node on mainnet and testnets.
                        Defaults to http://127.0.0.1:8545.")
                 .takes_value(true)
         )

beacon_node/src/config.rs

@@ -230,17 +230,14 @@ pub fn get_config<E: EthSpec>(
         );
         client_config.sync_eth1_chain = true;
 
-        let endpoints = vec![SensitiveUrl::parse(endpoint)
-            .map_err(|e| format!("eth1-endpoint was an invalid URL: {:?}", e))?];
-        client_config.eth1.endpoints = Eth1Endpoint::NoAuth(endpoints);
-    } else if let Some(endpoints) = cli_args.value_of("eth1-endpoints") {
+        let endpoint = SensitiveUrl::parse(endpoint)
+            .map_err(|e| format!("eth1-endpoint was an invalid URL: {:?}", e))?;
+        client_config.eth1.endpoint = Eth1Endpoint::NoAuth(endpoint);
+    } else if let Some(endpoint) = cli_args.value_of("eth1-endpoints") {
         client_config.sync_eth1_chain = true;
-        let endpoints = endpoints
-            .split(',')
-            .map(SensitiveUrl::parse)
-            .collect::<Result<_, _>>()
+        let endpoint = SensitiveUrl::parse(endpoint)
             .map_err(|e| format!("eth1-endpoints contains an invalid URL {:?}", e))?;
-        client_config.eth1.endpoints = Eth1Endpoint::NoAuth(endpoints);
+        client_config.eth1.endpoint = Eth1Endpoint::NoAuth(endpoint);
     }
 
     if let Some(val) = cli_args.value_of("eth1-blocks-per-log-query") {
@@ -326,7 +323,7 @@ pub fn get_config<E: EthSpec>(
                     --eth1-endpoints has been deprecated for post-merge configurations"
             );
         }
-        client_config.eth1.endpoints = Eth1Endpoint::Auth {
+        client_config.eth1.endpoint = Eth1Endpoint::Auth {
             endpoint: execution_endpoint,
             jwt_path: secret_file,
             jwt_id: el_config.jwt_id.clone(),

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