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)
         )