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>

common/eth2/src/lib.rs

@@ -1508,7 +1508,7 @@ impl BeaconNodeHttpClient {
 
 /// Returns `Ok(response)` if the response is a `200 OK` response. Otherwise, creates an
 /// appropriate error message.
-async fn ok_or_error(response: Response) -> Result<Response, Error> {
+pub async fn ok_or_error(response: Response) -> Result<Response, Error> {
     let status = response.status();
 
     if status == StatusCode::OK {