Prune CBS virt7 repository provisioning for CentOS Docker servers

Summary:
Previously, Docker and other virt technology packages was provided
by the virt7 cbs repository on CentOS, per T45.

Docker is now installed through the docker-ce repository,
directly provided by Docker.

This is a follow-up for b2d950dd11a7.

This change reverts acddcae6a8d6.

Test Plan:
This cleaning change is considered no op:

• no op on Equatower, as the deployment was manual
• Dwellers use both repositories, but lxc is decom
• Future installations will also use docker-ce only

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: http://devcentral.nasqueron.org/D1751