Remove OpenShiftMockServer

[manusa]

Description

OpenShiftMockServer is deprecated since v6.0.0 (#3913), we should completely remove this class.

Originally posted in #5349

[stale]

This issue has been automatically marked as stale because it has not had any activity since 90 days. It will be closed if no further activity occurs within 7 days. Thank you for your contributions!

[shawkins]

@manusa based upon our recent conversations, do you want to hold off on this until 7? Or do you feel okay about removing deprecated test stuff in a minor release?

[manusa]

Hey @shawkins, sorry for the extremely belated reply.
Let's hold on until we clarify what release strategy we want to adopt.