Optimisations relating to get_expected_withdrawals (#9314)

Tweaking/optimising `get_expected_withdrawals` on the pre-Gloas path, after the removal of some optimisations in:

- https://github.com/sigp/lighthouse/pull/9102


  - Remove `prepare_beacon_proposer` calls from `register_validator`. These calls were: heavily duplicated, happening _too early_ (at the start of the slot before the block arrived), expensive on epoch boundaries (forcing an early epoch transition), _and_ unnecessary. They are completely safe to remove because the scheduled proposer preparation routine exists.
- Instrument `get_expected_withdrawals`/`prepare_beacon_proposer` to find more unexpected callers.


Co-Authored-By: Michael Sproul <michael@sigmaprime.io>

Co-Authored-By: Tan Chee Keong <tanck@sigmaprime.io>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

beacon_node/http_api/src/validator/mod.rs

@@ -664,6 +664,8 @@ pub fn post_validator_register_validator<T: BeaconChainTypes>(
                             .unzip();
 
                         // Update the prepare beacon proposer cache based on this request.
+                        // This data will get picked up by the next scheduled run of
+                        // `prepare_beacon_proposer`.
                         execution_layer
                             .update_proposer_preparation(
                                 current_epoch,
@@ -671,19 +673,6 @@ pub fn post_validator_register_validator<T: BeaconChainTypes>(
                             )
                             .await;
 
-                        // Call prepare beacon proposer blocking with the latest update in order to make
-                        // sure we have a local payload to fall back to in the event of the blinded block
-                        // flow failing.
-                        chain
-                            .prepare_beacon_proposer(current_slot)
-                            .await
-                            .map_err(|e| {
-                                warp_utils::reject::custom_bad_request(format!(
-                                    "error updating proposer preparations: {:?}",
-                                    e
-                                ))
-                            })?;
-
                         info!(
                             count = filtered_registration_data.len(),
                             "Forwarding register validator request to connected builder"