Deduplicate block root computation (#3590)

## Issue Addressed

NA

## Proposed Changes

This PR removes duplicated block root computation.

Computing the `SignedBeaconBlock::canonical_root` has become more expensive since the merge as we need to compute the merke root of each transaction inside an `ExecutionPayload`.

Computing the root for [a mainnet block](https://beaconcha.in/slot/4704236) is taking ~10ms on my i7-8700K CPU @ 3.70GHz (no sha extensions). Given that our median seen-to-imported time for blocks is presently 300-400ms, removing a few duplicated block roots (~30ms) could represent an easy 10% improvement. When we consider that the seen-to-imported times include operations *after* the block has been placed in the early attester cache, we could expect the 30ms to be more significant WRT our seen-to-attestable times.

## Additional Info

NA

beacon_node/beacon_chain/tests/tests.rs

@@ -685,6 +685,7 @@ async fn run_skip_slot_test(skip_slots: u64) {
         harness_b
             .chain
             .process_block(
+                harness_a.chain.head_snapshot().beacon_block_root,
                 harness_a.chain.head_snapshot().beacon_block.clone(),
                 CountUnrealized::True
             )