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/client/src/builder.rs

@@ -724,7 +724,7 @@ where
                     execution_layer.spawn_watchdog_routine(beacon_chain.slot_clock.clone());
 
                     // Spawn a routine that removes expired proposer preparations.
-                    execution_layer.spawn_clean_proposer_caches_routine::<TSlotClock, TEthSpec>(
+                    execution_layer.spawn_clean_proposer_caches_routine::<TSlotClock>(
                         beacon_chain.slot_clock.clone(),
                     );