panel/acme-domains: fix cyclic dependency in view model

panel/acme-domains: fix cyclic dependency in view model

A view model forumla cannot depend on itself, as that is a dependency
cycle.

In this specific case we can just drop the `hasUsage` forumla entry
completely, it is bogus as it was just returning its value, and that
is actually taken care of by the `hasUsage` data binding.

The debug build of ExtJS throws an exception on such cycles, the
release build does not cares to much and seems to do the right thing
already here.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>