fix OVN LS/LB gc

[zhangzujian]

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

What type of this PR

• Bug fixes

Which issue(s) this PR fixes:

Fixes LB gc for subnets not implemented by Kube-OVN:

I1121 18:49:31.261612       1 gc.go:293] start to gc loadbalancers
W1121 18:49:31.269664       1 ovn-nbctl.go:48] ovn-nbctl command error: ovn-nbctl --timeout=60 --no-wait --if-exists ls-lb-del macvlan-1 cluster-tcp-loadbalancer in 3ms
E1121 18:49:31.269737       1 ovn-nbctl.go:331] failed to add tcp lb to macvlan-1, ovn-nbctl: udp-st-info-dep: switch name not found
, "exit status 1"
F1121 18:49:31.269753       1 controller.go:387] gc failed: ovn-nbctl: macvlan-1: switch name not found
, "exit status 1"