add policy route when use old active gateway node for centralized subnet

[hongzhen-ma]

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

What type of this PR

• Bug fixes

Which issue(s) this PR fixes:

Fixes #(issue-number)

WHAT

🤖 Generated by Copilot at 905d29b

Improve policy route handling for centralized subnets with ecmp. Remove activateGateway field from subnet status and use node's tunnelIP as next hop.

🤖 Generated by Copilot at 905d29b

We're sailing on the network sea, with policy routes to guide us
We don't need activateGateway, we've got tunnelIP beside us
So haul away the subnet status, and make the ecmp steady
We'll reach the centralized subnets, with our code review ready

HOW

🤖 Generated by Copilot at 905d29b

• Remove the update of subnet status with activate gateway field for ecmp subnets (link)
• Use tunnel IP as next hop for policy route for centralized subnets (link)

[github-actions]

• The commit message should be more descriptive and informative.
• There is a commented out code block that needs to be removed.
• The logic for handling the ActivateGateway field when ecmp is enabled needs to be reviewed.

[github-actions]

• The commit message is missing. Please add a descriptive commit message.
• There is a potential bug in the code. When subnet.Spec.EnableEcmp is true, the ActivateGateway field is deleted from subnet.Status. However, if subnet.Spec.EnableEcmp is changed back to false, the ActivateGateway field is not added back to subnet.Status, which could cause incorrect behavior. This should be fixed by adding the ActivateGateway field back to subnet.Status when subnet.Spec.EnableEcmp is changed back to false.
• There are format errors in the code. Please run a linter or formatter to fix them.
• There are no performance issues in the code.
• There are ways to improve the code. Instead of using time.Sleep(1 * time.Minute) in the test, it would be better to use a WaitUntil function to wait for the policy route to be updated.