Remove a no-train registration (the same JSON body as POST; never a query string, which access logs keep). An API
key removes its own end users’ registrations, not one an admin made; an admin removes any: for a named client, or
with none in every scope (each configured key, the keyless one, every scope a registration is stored in and the
every-client one). While an admin’s every-client registration stays, the other scopes report kept. Without API
keys this needs an admin key in live mode, so nobody can switch storage back on for someone else’s users; the
mock demo’s keyless callers get no status, as they get no created from POST.