Remove app_server_type from top level usage ping

In #219114 (closed) we fixed the problem that the current app_server_type is not correctly reported.

With that change, we are also changing this element structurally and move it out of the top-level usage ping object and into the topology element, where we can track it per every web server node.

This MR is to remove the top-level app_server_type element from usage ping object, because it is not used anymore.

The co-responding changes in versions application is: https://gitlab.com/gitlab-services/version-gitlab-com/-/merge_requests/506

Close #225632 (closed)