update policy route when change from ecmp to active-standby

[hongzhen-ma]

• Make sure you have followed Kube-OVN Code Style.

What type of this PR

• Bug fixes

集中式子网由 ecmp 模式，切换为主备模式，需要删除非activeGateway 网关节点的策略路由

Which issue(s) this PR fixes:

Fixes #(issue-number)

WHAT

🤖 Generated by Copilot at cb282d6

This pull request enhances the support for ECMP routing for centralized subnets in kube-ovn. It modifies the checkPolicyRouteExistForNode function in node.go and the routing logic in subnet.go to handle different ECMP scenarios.

🤖 Generated by Copilot at cb282d6

Sing, O Muse, of the clever coder who devised
A better way to route the packets of the cloud,
With checkPolicyRouteExistForNode he revised
The function to support ECMP, of which he was proud.

HOW

🤖 Generated by Copilot at cb282d6

• Modify checkPolicyRouteExistForNode to return next hops and node name-ip map for ECMP support (link)
• Modify checkPolicyRouteExistForNode to check policy route existence based on ECMP configuration (link)
• Delete ActivateGateway field from subnet status when ECMP is enabled in reconcileOvnRoute (link)
• Add active-backup policy route for centralized subnet when ECMP is disabled in reconcileOvnRoute (link)