Revert "IGNITE-28477 Remove mutable path"

This reverts commit 7815031.

Author: chesnokoff

modules/core/src/main/java/org/apache/ignite/internal/processors/continuous/AbstractContinuousMessage.java

@@ -21,7 +21,6 @@
 import org.apache.ignite.internal.Order;
 import org.apache.ignite.internal.managers.discovery.DiscoveryCustomMessage;
 import org.apache.ignite.lang.IgniteUuid;
-import org.jetbrains.annotations.Nullable;
 
 /**
  *
@@ -51,9 +50,4 @@ protected AbstractContinuousMessage(UUID id) {
     public UUID routineId() {
         return routineId;
     }
-
-    /** {@inheritDoc} */
-    @Nullable @Override public DiscoveryCustomMessage ackMessage() {
-        return null;
-    }
 }