Merge Engines and Engine struct in one in the execution_layer crate (#3284)

## Issue Addressed

Part of https://github.com/sigp/lighthouse/issues/3118, continuation of https://github.com/sigp/lighthouse/pull/3257 and https://github.com/sigp/lighthouse/pull/3283

## Proposed Changes
- Merge the [`Engines`](9c429d0764/beacon_node/execution_layer/src/engines.rs (L161-L165)) struct and [`Engine` ](9c429d0764/beacon_node/execution_layer/src/engines.rs (L62-L67))
- Remove unnecessary generics 

## Additional Info
There is more cleanup to do that will come in subsequent PRs

beacon_node/execution_layer/src/engine_api.rs

@@ -71,8 +71,6 @@ impl From<builder_client::Error> for Error {
     }
 }
 
-pub struct EngineApi;
-
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum PayloadStatusV1Status {
     Valid,