fix(containerservice): remove preview API fields from agent pool implementation

- Removed nodeImageVersion field from agent pool inner model
- Removed upgradeStrategy field from agent pool inner model
- Cleaned up preview API version specific code
- Ensured compatibility with stable API versions

Author: v-huizhu2

sdk/containerservice/azure-resourcemanager-containerservice/src/main/java/com/azure/resourcemanager/containerservice/implementation/KubernetesClusterAgentPoolImpl.java

@@ -325,10 +325,6 @@ AgentPoolInner getAgentPoolInner() {
         agentPoolInner.withVirtualMachineNodesStatus(innerModel().virtualMachineNodesStatus());
         agentPoolInner.withStatus(innerModel().status());
 
-        // available only in preview api-version
-        agentPoolInner.withNodeImageVersion(innerModel().nodeImageVersion());
-        agentPoolInner.withUpgradeStrategy(innerModel().upgradeStrategy());
-
         return agentPoolInner;
     }