Remove OpenStack cloud feat reference

[lethedata]

Remove OpenStack reference on Cloud features page as OpenStack nova compute no longer supports the xen hypervisor.

Before submitting the pull request, you must agree with the following statements by checking both boxes with a 'x'.

• "I accept that my contribution is placed under the CC BY-SA 2.0 license [1]."
• "My contribution complies with the Developer Certificate of Origin [2]."

[1] https://creativecommons.org/licenses/by-sa/2.0/
[2] https://docs.xcp-ng.org/project/contributing/#developer-certificate-of-origin-dco

[lethedata]

Xen no longer shows up in nova's support-matrix.ini. There's also a lot of xen depreciation/removal notices in the release notes such as xenapi driver from deprecate-xen-driver-train-bd57a16fa51ab679.yaml.