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/eth1/src/inner.rs

@@ -1,14 +1,14 @@
+use crate::service::endpoint_from_config;
 use crate::Config;
 use crate::{
     block_cache::{BlockCache, Eth1Block},
     deposit_cache::{DepositCache, SszDepositCache},
-    service::EndpointsCache,
 };
+use execution_layer::HttpJsonRpc;
 use parking_lot::RwLock;
 use ssz::four_byte_option_impl;
 use ssz::{Decode, Encode};
 use ssz_derive::{Decode, Encode};
-use std::sync::Arc;
 use types::ChainSpec;
 
 // Define "legacy" implementations of `Option<u64>` which use four bytes for encoding the union
@@ -31,11 +31,10 @@ impl DepositUpdater {
     }
 }
 
-#[derive(Default)]
 pub struct Inner {
     pub block_cache: RwLock<BlockCache>,
     pub deposit_cache: RwLock<DepositUpdater>,
-    pub endpoints_cache: RwLock<Option<Arc<EndpointsCache>>>,
+    pub endpoint: HttpJsonRpc,
     pub config: RwLock<Config>,
     pub remote_head_block: RwLock<Option<Eth1Block>>,
     pub spec: ChainSpec,
@@ -96,7 +95,8 @@ impl SszEth1Cache {
                 cache: self.deposit_cache.to_deposit_cache()?,
                 last_processed_block: self.last_processed_block,
             }),
-            endpoints_cache: RwLock::new(None),
+            endpoint: endpoint_from_config(&config)
+                .map_err(|e| format!("Failed to create endpoint: {:?}", e))?,
             // Set the remote head_block zero when creating a new instance. We only care about
             // present and future eth1 nodes.
             remote_head_block: RwLock::new(None),