Remove cache operations from epoch processing.

- Don't build the next cache at all.
- Call `advance_caches()` in per-slot processing.

eth2/state_processing/src/epoch_processable.rs

@@ -619,12 +619,6 @@ impl EpochProcessable for BeaconState {
             .cloned()
             .collect();
 
-        /*
-         * Manage the beacon state caches
-         */
-        self.advance_caches();
-        self.build_epoch_cache(RelativeEpoch::Next, spec)?;
-
         debug!("Epoch transition complete.");
 
         Ok(())