clean up code about static routes

[zhangzujian]

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

What type of this PR

Examples of user facing changes:

• Features
• Bug fixes
• Docs
• Tests

Which issue(s) this PR fixes:

Fixes #(issue-number)

WHAT

🤖 Generated by Copilot at 534bb67

This pull request refactors the Controller struct to remove the static routing logic for subnets with centralized gateways. This reduces the complexity and potential errors of the routing management in pkg/controller/controller.go and pkg/controller/subnet.go.

🤖 Generated by Copilot at 534bb67

Controller cleans up
No more static routes for subnets
Autumn leaves fall fast

HOW

🤖 Generated by Copilot at 534bb67

• Remove deleteRouteQueue field from Controller struct and its initialization and shutdown in Run and shutdown methods (link, link, link)
• Remove invocation of runDeleteRouteWorker method in startWorkers method of Controller struct (link)
• Remove conditional logic that adds a subnet to deleteRouteQueue in enqueueDeleteSubnet method of Controller struct (link)
• Remove definition of runDeleteRouteWorker method and its helper methods processNextDeleteRoutePodWorkItem and handleDeleteRoute from subnet.go file (link, link, link)
• Remove invocation of deleteStaticRoute method in reconcileOvnDefaultVpcRoute method of Controller struct (link)

[github-actions]

• The deleteRouteQueue has been removed from the Controller struct, but it is still being used in the enqueueDeleteSubnet function. This should be updated to remove the call to c.deleteRouteQueue.Add(obj).
• The runDeleteRouteWorker function has been removed, but it is still being called in the startWorkers function. This should be updated to remove the call to go wait.Until(c.runDeleteRouteWorker, time.Second, ctx.Done()).
• The handleDeleteRoute function has been removed, but it is still being called in the processNextDeleteRoutePodWorkItem function. This should be updated to remove the call to if err := c.handleDeleteRoute(subnet); err != nil {.
• The handleUpdateSubnetStatus function is calling c.subnetsLister.Get, which could potentially cause performance issues if there are a large number of subnets. Consider using a cache instead.
• The reconcileOvnDefaultVpcRoute function is deleting a static route without checking if it exists first. This could result in an error if the route does not exist. Consider adding a check to see if the route exists before attempting to delete it.