[KNI] Dockerfile: drop python2

[swatisehgal]

Python2 no longer needed and its presence is causing CI failure in #116.

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: swatisehgal

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[ffromani]

/approve
/lgtm

thanks!

[openshift-ci]

@swatisehgal: 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/ci-unit	b640f8d	link	true	/test ci-unit

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