Remove CentOS from helix-machines images

[dougbu]

We currently have six centos.7.amd64* Helix queues, none of which have been used since dotnet/performance stopped testing their release/7.0 branch back in July. It's time to remove these queues.

As background, note CentOS 7 is out of support. See https://www.redhat.com/en/blog/centos-linux-has-reached-its-end-life-eol and note the lack of extended support options in https://www.redhat.com/en/topics/linux/centos-linux-eol

Release Note Category

• Feature changes/additions
• Bug fixes
• Internal Infrastructure Improvements

Release Note Description

Removed the following Helix queues: centos.7.amd64, centos.7.amd64.open, centos.7.amd64.open.rt, centos.7.amd64,open.svc, centos.7.amd64.rt, and centos.7.amd64.svc due to lack of use and the EOL status of the operating system.