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/test_utils.rs

@@ -147,7 +147,7 @@ pub struct Builder<T: BeaconChainTypes> {
     store: Option<Arc<HotColdDB<T::EthSpec, T::HotStore, T::ColdStore>>>,
     initial_mutator: Option<BoxedMutator<T::EthSpec, T::HotStore, T::ColdStore>>,
     store_mutator: Option<BoxedMutator<T::EthSpec, T::HotStore, T::ColdStore>>,
-    execution_layer: Option<ExecutionLayer>,
+    execution_layer: Option<ExecutionLayer<T::EthSpec>>,
     mock_execution_layer: Option<MockExecutionLayer<T::EthSpec>>,
     runtime: TestRuntime,
     log: Logger,
@@ -361,6 +361,7 @@ where
             DEFAULT_TERMINAL_BLOCK,
             spec.terminal_block_hash,
             spec.terminal_block_hash_activation_epoch,
+            None,
         );
         self.execution_layer = Some(mock.el.clone());
         self.mock_execution_layer = Some(mock);