remove static route for custom vpc

[zcq98]

Feature request

Default static route will be set if vpc enables external subnet.

192.168.80.1 is the gateway of subnet external

extra static route should be set if using extra external subnets referring to OVN EIP FIP SNAT

Use case

Actually,there is no need to Configure static routes with multiple external subnets.
for the bugfix: kubeovn/ovn#4,ovn will set the logical flow according to which external subnet the eip ip belongs to:

external ext1 ext2 are external subnets connect to vpc1,the outport will be set according to eip ip.
other subnets will be automatically routed according to the external subnet where the eip is located.
thus there is no need to add static route for custom vpc.