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: 5a3bcd2
- The last Lighthouse version: v1.5.1

Author: michaelsproul

Cargo.lock

@@ -33,7 +33,6 @@ members = [
     "common/logging",
     "common/lru_cache",
     "common/malloc_utils",
-    "common/remote_signer_consumer",
     "common/sensitive_url",
     "common/slot_clock",
     "common/task_executor",
@@ -69,17 +68,12 @@ members = [
     "lighthouse",
     "lighthouse/environment",
 
-    "remote_signer",
-    "remote_signer/backend",
-    "remote_signer/client",
-
     "slasher",
     "slasher/service",
 
     "testing/ef_tests",
     "testing/eth1_test_rig",
     "testing/node_test_rig",
-    "testing/remote_signer_test",
     "testing/simulator",
     "testing/state_transition_vectors",