Don't delete the routes which are added for the peer IPv6 gateways

[Jexf]

Describe the bug

"on-link" is not identified in IPv6 route entries, the peer IPv6 gateways need to add into IPv6 routes separately. But the related routes will be removed in init node routes process when Antrea Agent starting, then sync again.

I0228 02:10:02.574234       1 route_linux.go:730] Deleting unknown route {Ifindex: 7 Dst: fc00::1/128 Src: <nil> Gw: <nil> Flags: [] Table: 254}
I0228 02:10:02.574432       1 route_linux.go:730] Deleting unknown route {Ifindex: 7 Dst: fc00:0:0:2::1/128 Src: <nil> Gw: <nil> Flags: [] Table: 254}
I0228 02:10:02.574900       1 node_route_controller.go:546] "Adding route and flow to Node" Node="tos-04" podCIDR="fc00::/64" peerNodeIP="2003:ac18::30a:15"
I0228 02:10:02.575073       1 node_route_controller.go:546] "Adding route and flow to Node" Node="tos-06" podCIDR="fc00:0:0:2::/64" peerNodeIP="2003:ac18::30a:17"

Add peer IPv6 gateways routes logic:

		if podCIDR.IP.To4() == nil {
			// "on-link" is not identified in IPv6 route entries, so split the configuration into 2 entries.
			routes = []*netlink.Route{
				{
					Dst:       &net.IPNet{IP: nodeGwIP, Mask: net.IPMask{0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}},
					LinkIndex: c.nodeConfig.GatewayConfig.LinkIndex,
				},
			}
		} else {
			route.Flags = int(netlink.FLAG_ONLINK)
		}

To Reproduce
1.Use dual-stack with Antrea
2.Restart Antrea Agent

Expected
Just remove the dirty routes, and the routes which have been added for peer IPv6 gateways need to be reserved

Actual behavior

Remove the related routes, then add again.

Versions:
Antrea 1.5.0