Drop unused import

Author: yevhenii.nadtochii

server/src/test/java/io/spine/server/delivery/given/DeliveryTestEnv.java

@@ -38,7 +38,6 @@
 import io.spine.server.delivery.InboxMessage;
 import io.spine.server.delivery.ShardIndex;
 import io.spine.server.delivery.ShardObserver;
-import io.spine.server.delivery.WorkerId;
 import io.spine.server.route.EventRoute;
 import io.spine.server.route.EventRouting;