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/beacon_chain/src/builder.rs

@@ -77,7 +77,7 @@ pub struct BeaconChainBuilder<T: BeaconChainTypes> {
     >,
     op_pool: Option<OperationPool<T::EthSpec>>,
     eth1_chain: Option<Eth1Chain<T::Eth1Chain, T::EthSpec>>,
-    execution_layer: Option<ExecutionLayer>,
+    execution_layer: Option<ExecutionLayer<T::EthSpec>>,
     event_handler: Option<ServerSentEventHandler<T::EthSpec>>,
     slot_clock: Option<T::SlotClock>,
     shutdown_sender: Option<Sender<ShutdownReason>>,
@@ -481,7 +481,7 @@ where
     }
 
     /// Sets the `BeaconChain` execution layer.
-    pub fn execution_layer(mut self, execution_layer: Option<ExecutionLayer>) -> Self {
+    pub fn execution_layer(mut self, execution_layer: Option<ExecutionLayer<TEthSpec>>) -> Self {
         self.execution_layer = execution_layer;
         self
     }