Remove unused checkpoint metrics (MystenLabs#12863)

## Description 

as title

## Test Plan 

eyes

---
If your changes are not user-facing and not a breaking change, you can
skip the following section. Otherwise, please indicate what changed, and
then add to the Release Notes section as highlighted during the release
process.

### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes
Remove unused checkpoint metrics

crates/sui-core/src/checkpoints/metrics.rs

@@ -13,8 +13,6 @@ pub struct CheckpointMetrics {
     pub last_certified_checkpoint: IntGauge,
     pub last_constructed_checkpoint: IntGauge,
     pub checkpoint_errors: IntCounter,
-    pub builder_utilization: IntCounter,
-    pub aggregator_utilization: IntCounter,
     pub transactions_included_in_checkpoint: IntCounter,
     pub checkpoint_roots_count: IntCounter,
     pub checkpoint_participation: IntCounterVec,
@@ -58,18 +56,6 @@ impl CheckpointMetrics {
                 registry
             )
             .unwrap(),
-            builder_utilization: register_int_counter_with_registry!(
-                "builder_utilization",
-                "Checkpoints builder task utilization",
-                registry
-            )
-            .unwrap(),
-            aggregator_utilization: register_int_counter_with_registry!(
-                "aggregator_utilization",
-                "Checkpoints aggregator task utilization",
-                registry
-            )
-            .unwrap(),
             transactions_included_in_checkpoint: register_int_counter_with_registry!(
                 "transactions_included_in_checkpoint",
                 "Transactions included in a checkpoint",