Fix compilation, clear best_child/best_descendant in migration

- Fix leaf detection in heads_descended_from_finalization (parent()
  method call, map away enumerate index)
- Clear best_child and best_descendant in v28->v29 migration (no
  longer used, replaced by virtual tree walk)
- Migration now rewrites fork choice data instead of being a no-op

consensus/fork_choice/src/fork_choice.rs

@@ -1008,8 +1008,6 @@ where
                 proposer_index: Some(block.proposer_index()),
             },
             current_slot,
-            self.justified_checkpoint(),
-            self.finalized_checkpoint(),
             spec,
             block_delay,
         )?;