mgr/cephadm/tests/test_cephadm: remove broken rgw_update_fail test

mgr/cephadm/tests/test_cephadm: remove broken rgw_update_fail test

This was trying to show that we won't put 2 RGWs from different zones
on the same host.  In reality, the test was only passing because it was
specifying an invalid placement spec (count=3 in a 2-host cluster).

It's true that we shouldn't do that because they won't be able to bind,
but the HostAssignment code doesn't handle this case currently.

See https://tracker.ceph.com/issues/44397

Signed-off-by: Sage Weil <sage@redhat.com>