Remove Windows 1903/1909 machines/config

Not to be done before 9th August 2021

As part of gitlab-org/gitlab-runner#27551 (closed), we're deprecating our image building for 1903.

• Remove 1903/1909 build machines
• Remove autoscaler installation in https://ops.gitlab.net/gitlab-com/gl-infra/ci-infrastructure-windows/-/blob/c065a7aa04c958ffe941c0a80fbd4dba592e0c1a/ansible/group_vars/gcp_role_runner_manager.yml#L44-69

We shouldn’t remove these yet, as they may be required for 13.11 and 13.12 security releases.