deprecate 2015 IT cert

[Ximinhan]

2015 cert is deprecated on https://certs.corp.redhat.com/certs/

[locriandev]

/lgtm

[openshift-ci]

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[openshift-ci]

@Ximinhan: 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
ci/prow/security	61d1afa	link	false	/test security

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