[v9] revert #9540 (static role check for access request deletion)

[espadolini]

This PR reverts #9540. As #9540 was never backported at the time, it's only in master and v9. (see #11220)

The original intent behind #9540 was to mitigate the lack of visibility over access request deletions, as a malicious actor who was able to escalate permissions through access requests in such a way that they were also granted deletion permissions could acquire credentials and then delete the access requests used to hide their tracks.

After #9758 and #9552 this concern no longer exists, and (as @nklaassen found out) the way #9540 is implemented breaks tctl requests rm (because the system user used by tctl with the Admin role is not a user that exists in the backend), so the cleanest solution is to remove this special case altogether.