[Google | refactor] Remove backoff_if_unfound method from google compute service

The better way to wait until resource is created is to check if operation object that is return by API call is ready. Since we use this approach in other places, I've decided to remove usage `backoff_if_unfound`. Also `backoff_if_unfound` was complication and wasn't tested.

It could be a better way of getting `operation` object, but I've done it this way using other models as an example.

I've run manually mock and live tests (live tests was take from #2995): all tests passed.