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/tests/payload_invalidation.rs

@@ -64,7 +64,7 @@ impl InvalidPayloadRig {
         self
     }
 
-    fn execution_layer(&self) -> ExecutionLayer {
+    fn execution_layer(&self) -> ExecutionLayer<E> {
         self.harness.chain.execution_layer.clone().unwrap()
     }