Remove deprecated KUBECONFIG CI variable

Since unit tests no longer depend on an active cluster with #221 (closed), we should be able to remove the KUBECONFIG CI variable in this project.

This variable is confusing because we connect our OpenShift and GKE CI clusters with KUBECONFIG_OCP_* and KUBECONFIG_GKE CI variables. KUBECONFIG says nothing about what cluster to which it connects, so I propose we remove this variable, or rename it if it is still required.