do not leak LB when ingress class is changed [release-1.4]

[agau4779]

Cherry-picking #547

[k8s-ci-robot]

Hi @agau4779. Thanks for your PR.

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[agau4779]

/assign @rramkumar1

[rramkumar1]

/lgtm

[rramkumar1]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: agau4779, rramkumar1

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