[release-1.0] fix: skip instance not found when decoupling vmss from lb

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #842

/assign MartinForReal

fix: skip instance not found when decoupling vmss from lb

[k8s-ci-robot]

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[k8s-ci-robot]

@k8s-infra-cherrypick-robot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cloud-provider-azure-unit-1-21	9dead81	link	true	/test pull-cloud-provider-azure-unit-1-21

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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