Remove auto_release_flood_stage_block property check

This property was deprecated in elastic#45274 (8.0)

Author: arteam

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

@@ -156,19 +156,6 @@ 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));