Archive remote_signer code (#2559)

## Proposed Changes

This PR deletes all `remote_signer` code from Lighthouse, for the following reasons:

* The `remote_signer` code is unused, and we have no plans to use it now that we're moving to supporting the Web3Signer APIs: #2522
* It represents a significant maintenance burden. The HTTP API tests have been prone to platform-specific failures, and breakages due to dependency upgrades, e.g. #2400.

Although the code is deleted it remains in the Git history should we ever want to recover it. For ease of reference:

- The last commit containing remote signer code: 5a3bcd2904
- The last Lighthouse version: v1.5.1

lighthouse/src/main.rs

@@ -164,7 +164,6 @@ fn main() {
         .subcommand(boot_node::cli_app())
         .subcommand(validator_client::cli_app())
         .subcommand(account_manager::cli_app())
-        .subcommand(remote_signer::cli_app())
         .get_matches();
 
     // Configure the allocator early in the process, before it has the chance to use the default values for
@@ -394,16 +393,6 @@ fn run<E: EthSpec>(
                 ));
             }
         }
-        ("remote_signer", Some(matches)) => {
-            if let Err(e) = remote_signer::run(&mut environment, matches) {
-                crit!(log, "Failed to start remote signer"; "reason" => e);
-                let _ = environment
-                    .core_context()
-                    .executor
-                    .shutdown_sender()
-                    .try_send(ShutdownReason::Failure("Failed to start remote signer"));
-            }
-        }
         _ => {
             crit!(log, "No subcommand supplied. See --help .");
             return Err("No subcommand supplied.".into());