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/genesis/tests/tests.rs

@@ -44,10 +44,9 @@ fn basic() {
 
         let service = Eth1GenesisService::new(
             Eth1Config {
-                endpoints: Eth1Endpoint::NoAuth(vec![SensitiveUrl::parse(
-                    eth1.endpoint().as_str(),
-                )
-                .unwrap()]),
+                endpoint: Eth1Endpoint::NoAuth(
+                    SensitiveUrl::parse(eth1.endpoint().as_str()).unwrap(),
+                ),
                 deposit_contract_address: deposit_contract.address(),
                 deposit_contract_deploy_block: now,
                 lowest_cached_block_number: now,
@@ -57,7 +56,8 @@ fn basic() {
             },
             log,
             spec.clone(),
-        );
+        )
+        .unwrap();
 
         // NOTE: this test is sensitive to the response speed of the external web3 server. If
         // you're experiencing failures, try increasing the update_interval.