issue_triage.py should not consider labels that have been removed

[jlewi]

Right now if a label is added and then removed issue_triage.py would still take into account that label when deciding whether an issue needs triage.

This is because we look at the time labels were created: https://github.com/kubeflow/code-intelligence/blob/ecbab8b8ca38ce8175c0a2550ab2a916e14d0e1d/py/issue_triage/triage.py#L81

But we don't take into account whether that label is still present on the issue.

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[jtfogarty]

/area engprod /priority p2

[mukul-mehta]

Could I work on this issue? I'm planning to contribute to Kubeflow during GSoC '20. Could you please also add me to the slack channel?