Remove builder redundancy (#3294)

## Issue Addressed

This PR is a subset of the changes in #3134. Unstable will still not function correctly with the new builder spec once this is merged, #3134 should be used on testnets

## Proposed Changes

- Removes redundancy in "builders" (servers implementing the builder spec)
- Renames `payload-builder` flag to `builder`
- Moves from old builder RPC API to new HTTP API, but does not implement the validator registration API (implemented in https://github.com/sigp/lighthouse/pull/3194)



Co-authored-by: sean <seananderson33@gmail.com>
Co-authored-by: realbigsean <sean@sigmaprime.io>

beacon_node/execution_layer/src/engine_api.rs

@@ -1,11 +1,9 @@
 use crate::engines::ForkChoiceState;
-use async_trait::async_trait;
 pub use ethers_core::types::Transaction;
 use http::deposit_methods::RpcError;
 pub use json_structures::TransitionConfigurationV1;
 use reqwest::StatusCode;
 use serde::{Deserialize, Serialize};
-use slog::Logger;
 pub use types::{
     Address, EthSpec, ExecutionBlockHash, ExecutionPayload, ExecutionPayloadHeader, FixedVector,
     Hash256, Uint256, VariableList,
@@ -28,10 +26,7 @@ pub enum Error {
     InvalidExecutePayloadResponse(&'static str),
     JsonRpc(RpcError),
     Json(serde_json::Error),
-    ServerMessage {
-        code: i64,
-        message: String,
-    },
+    ServerMessage { code: i64, message: String },
     Eip155Failure,
     IsSyncing,
     ExecutionBlockNotFound(ExecutionBlockHash),
@@ -40,15 +35,9 @@ pub enum Error {
     PayloadIdUnavailable,
     TransitionConfigurationMismatch,
     PayloadConversionLogicFlaw,
-    InvalidBuilderQuery,
-    MissingPayloadId {
-        parent_hash: ExecutionBlockHash,
-        timestamp: u64,
-        prev_randao: Hash256,
-        suggested_fee_recipient: Address,
-    },
     DeserializeTransaction(ssz_types::Error),
     DeserializeTransactions(ssz_types::Error),
+    BuilderApi(builder_client::Error),
 }
 
 impl From<reqwest::Error> for Error {
@@ -76,19 +65,14 @@ impl From<auth::Error> for Error {
     }
 }
 
-pub struct EngineApi;
-pub struct BuilderApi;
-
-#[async_trait]
-pub trait Builder {
-    async fn notify_forkchoice_updated(
-        &self,
-        forkchoice_state: ForkChoiceState,
-        payload_attributes: Option<PayloadAttributes>,
-        log: &Logger,
-    ) -> Result<ForkchoiceUpdatedResponse, Error>;
+impl From<builder_client::Error> for Error {
+    fn from(e: builder_client::Error) -> Self {
+        Error::BuilderApi(e)
+    }
 }
 
+pub struct EngineApi;
+
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum PayloadStatusV1Status {
     Valid,