docs: Remove Zun documentation with kata containers

This PR removes the zun documentation use case with kata containers mainly
because is not longer valid as it is using as a reference docker with
clear containers 2.0 which are not longer being supported and it is also
using docker to test kata with openstack zun and docker is also not supported.

Fixes kata-containers#3581

Signed-off-by: Gabriela Cervantes <[email protected]>

Author: GabyCT

docs/README.md

@@ -28,7 +28,6 @@ See the [howto documentation](how-to).
 ## Kata Use-Cases
 
 * [GPU Passthrough with Kata](./use-cases/GPU-passthrough-and-Kata.md)
-* [OpenStack Zun with Kata Containers](./use-cases/zun_kata.md)
 * [SR-IOV with Kata](./use-cases/using-SRIOV-and-kata.md)
 * [Intel QAT with Kata](./use-cases/using-Intel-QAT-and-kata.md)
 * [VPP with Kata](./use-cases/using-vpp-and-kata.md)