ceph-defaults: remove rgw from ceph_conf_overrides

ceph-defaults: remove rgw from ceph_conf_overrides

The [rgw] section in the ceph.conf file or via the ceph_conf_overrides
variable doesn't exist and has no effect.
To apply overrides to all radosgw instances we should use either the
[global] or [client] sections.
Overrides per radosgw instance should still use the
[client.rgw.{instance-name}] section.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1794552
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>