Remove auto_release_flood_stage_block property check (elastic#114696)

There's no option to disable the auto release of the write block when a node exceeds the flood-stage watermark.

This property was deprecated in elastic#45274 (8.0)

server/src/main/java/org/elasticsearch/cluster/routing/allocation/DiskThresholdSettings.java

@@ -19,7 +19,6 @@
 import org.elasticsearch.common.unit.RelativeByteSizeValue;
 import org.elasticsearch.core.Strings;
 import org.elasticsearch.core.TimeValue;
-import org.elasticsearch.core.UpdateForV9;
 
 import java.io.IOException;
 import java.util.Iterator;
@@ -156,19 +155,6 @@ public class DiskThresholdSettings implements Writeable {
     private volatile boolean enabled;
     private volatile TimeValue rerouteInterval;
 
-    static {
-        checkAutoReleaseIndexEnabled();
-    }
-
-    @UpdateForV9(owner = UpdateForV9.Owner.DISTRIBUTED_COORDINATION) // this check is unnecessary in v9
-    private static void checkAutoReleaseIndexEnabled() {
-        final String AUTO_RELEASE_INDEX_ENABLED_KEY = "es.disk.auto_release_flood_stage_block";
-        final String property = System.getProperty(AUTO_RELEASE_INDEX_ENABLED_KEY);
-        if (property != null) {
-            throw new IllegalArgumentException("system property [" + AUTO_RELEASE_INDEX_ENABLED_KEY + "] may not be set");
-        }
-    }
-
     public DiskThresholdSettings(Settings settings, ClusterSettings clusterSettings) {
         setLowWatermark(CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING.get(settings));
         setLowStageMaxHeadroom(CLUSTER_ROUTING_ALLOCATION_LOW_DISK_MAX_HEADROOM_SETTING.get(settings));