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/beacon_chain/src/canonical_head.rs

@@ -654,11 +654,11 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                     })
                 })
                 .and_then(|mut snapshot| {
-                    // Regardless of where we got the state from, attempt to build all the
-                    // caches except the tree hash cache.
+                    // Regardless of where we got the state from, attempt to build the committee
+                    // caches.
                     snapshot
                         .beacon_state
-                        .build_all_caches(&self.spec)
+                        .build_all_committee_caches(&self.spec)
                         .map_err(Into::into)
                         .map(|()| snapshot)
                 })?;