Remove PERMIT_HANDSHAKES_FROM_INCOMPATIBLE_BUILDS_KEY block (#113041)

This sysprop has been forbidden for all of 8.x, we don't need to even
think about it in 9.x. This commit removes it.

Author: DaveCTurner

server/src/main/java/org/elasticsearch/transport/TransportService.java

@@ -43,7 +43,6 @@
 import org.elasticsearch.core.Predicates;
 import org.elasticsearch.core.Releasable;
 import org.elasticsearch.core.TimeValue;
-import org.elasticsearch.core.UpdateForV9;
 import org.elasticsearch.node.NodeClosedException;
 import org.elasticsearch.node.ReportingService;
 import org.elasticsearch.tasks.Task;
@@ -1752,15 +1751,6 @@ Releasable withRef() {
         }
     }
 
-    static {
-        // Ensure that this property, introduced and immediately deprecated in 7.11, is not used in 8.x
-        @UpdateForV9 // we can remove this whole block in v9
-        final String PERMIT_HANDSHAKES_FROM_INCOMPATIBLE_BUILDS_KEY = "es.unsafely_permit_handshake_from_incompatible_builds";
-        if (System.getProperty(PERMIT_HANDSHAKES_FROM_INCOMPATIBLE_BUILDS_KEY) != null) {
-            throw new IllegalArgumentException("system property [" + PERMIT_HANDSHAKES_FROM_INCOMPATIBLE_BUILDS_KEY + "] must not be set");
-        }
-    }
-
     private record UnregisterChildTransportResponseHandler<T extends TransportResponse>(
         Releasable unregisterChildNode,
         TransportResponseHandler<T> handler,