[os-acc] os-acc will be retired as a project

A project called os-acc [1] was created in Stein cycle based on an expectation that it will be used for Cyborg - Nova integration. It is not relevant anymore and we have no plans to support it in Train.

We are discontinuing it with immediate effect. It was never used and had no developer base to speak of. So, we do not see any issues or impact for anybody.

[1] https://opendev.org/openstack/os-acc/

Regards, Sundar