remove cleanupkeys

Signed-off-by: Kiran Prakash <awskiran@amazon.com>

Author: kiranprakash154

server/src/main/java/org/opensearch/indices/IndicesRequestCache.java

@@ -140,7 +140,6 @@ public final class IndicesRequestCache extends AbstractLifecycleComponent
     private final static long BASE_RAM_BYTES_USED = RamUsageEstimator.shallowSizeOfInstance(Key.class);
 
     private final ConcurrentMap<CleanupKey, Boolean> registeredClosedListeners = ConcurrentCollections.newConcurrentMap();
-    private final Set<CleanupKey> keysToClean = ConcurrentCollections.newConcurrentSet();
     private final ByteSizeValue size;
     private final TimeValue expire;
     private final ICache<Key, BytesReference> cache;