Support IG backends removal in regional_ig_linker.go.

[mmamczur]

Tried to make it so that whatever exists in the backends list is preserved instead of overwriting with 'wantIGs'.

[k8s-ci-robot]

Hi @mmamczur. Thanks for your PR.

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[bowei]

/ok-to-test

[panslava]

code looks good to me, but still want @swetharepakula to take a look, I am not sure, if we should remove instance groups, and why it was not done before (in ig_linker.go as well)

[swetharepakula]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mmamczur, swetharepakula

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