Revert "Optimise HTTP validator lookups" (#3658)

## Issue Addressed

This reverts commit ca9dc8e094 (PR #3559) with some modifications.

## Proposed Changes

Unfortunately that PR introduced a performance regression in fork choice. The optimisation _intended_ to build the exit and pubkey caches on the head state _only if_ they were not already built. However, due to the head state always being cloned without these caches, we ended up building them every time the head changed, leading to a ~70ms+ penalty on mainnet.

fcfd02aeec/beacon_node/beacon_chain/src/canonical_head.rs (L633-L636)

I believe this is a severe enough regression to justify immediately releasing v3.2.1 with this change.

## Additional Info

I didn't fully revert #3559, because there were some unrelated deletions of dead code in that PR which I figured we may as well keep.

An alternative would be to clone the extra caches, but this likely still imposes some cost, so in the interest of applying a conservative fix quickly, I think reversion is the best approach. The optimisation from #3559 was not even optimising a particularly significant path, it was mostly for VCs running larger numbers of inactive keys. We can re-do it in the `tree-states` world where cache clones are cheap.

beacon_node/http_api/src/lib.rs

@@ -668,10 +668,9 @@ pub fn serve<T: BeaconChainTypes>(
                 "Invalid validator ID".to_string(),
             ))
         }))
-        .and(log_filter.clone())
         .and(warp::path::end())
         .and_then(
-            |state_id: StateId, chain: Arc<BeaconChain<T>>, validator_id: ValidatorId, log| {
+            |state_id: StateId, chain: Arc<BeaconChain<T>>, validator_id: ValidatorId| {
                 blocking_json_task(move || {
                     let (data, execution_optimistic) = state_id
                         .map_state_and_execution_optimistic(
@@ -679,23 +678,7 @@ pub fn serve<T: BeaconChainTypes>(
                             |state, execution_optimistic| {
                                 let index_opt = match &validator_id {
                                     ValidatorId::PublicKey(pubkey) => {
-                                        // Fast path: use the pubkey cache which is probably
-                                        // initialised at the head.
-                                        match state.get_validator_index_read_only(pubkey) {
-                                            Ok(result) => result,
-                                            Err(e) => {
-                                                // Slow path, fall back to iteration.
-                                                debug!(
-                                                    log,
-                                                    "Validator look-up cache miss";
-                                                    "reason" => ?e,
-                                                );
-                                                state
-                                                    .validators()
-                                                    .iter()
-                                                    .position(|v| v.pubkey == *pubkey)
-                                            }
-                                        }
+                                        state.validators().iter().position(|v| v.pubkey == *pubkey)
                                     }
                                     ValidatorId::Index(index) => Some(*index as usize),
                                 };