chore: Remove redundant KUBECONFIG env var

[tolusha]

What does this PR do

Remove redundant KUBECONFIG env var since kubeconfig is mounted as a Secret and env var is set by dashboard [1]

Reference issue

https://issues.redhat.com/browse/CRW-4332

[1] eclipse-che/che-dashboard#950

[l0rd]

LGTM 👍 cc @AObuchow

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[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: AObuchow, l0rd, tolusha

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[openshift-merge-robot]

PR needs rebase.

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