docs: Remove unnecessary <auth> example for iscsi disk type='volume'

docs: Remove unnecessary <auth> example for iscsi disk type='volume'

Alter the example to remove the <auth> from:

  <disk type='volume' device='disk'>
    <driver name='qemu' type='raw'/>
    <source pool='iscsi-pool' volume='unit:0:0:1' mode='host'/>
    <auth username='myuser'>
      <secret type='iscsi' usage='libvirtiscsi'/>
    </auth>
    <target dev='vdb' bus='virtio'/>
  </disk>

and

  <disk type='volume' device='disk'>
    <driver name='qemu' type='raw'/>
    <source pool='iscsi-pool' volume='unit:0:0:2' mode='direct'/>
    <auth username='myuser'>
      <secret type='iscsi' usage='libvirtiscsi'/>
    </auth>
    <target dev='vdc' bus='virtio'/>
  </disk>

The reality is, it's not even used. For a <source pool> the authdef
from the storage source pool will supercede whatever is in the <disk>
definition during virStorageTranslateDiskSourcePool processing. In fact,
if the pool doesn't have/need authentication, then the authdef would
be removed anyway as the storage pool would be handling things.

The "proof" for this is in the adjustment to the test to add an
<auth> for a disk. The resulting .args file won't add what normally
would be added "myname:encodedpassword@" prior to the hostname in
the IQN (e.g. iscsi://myname:encodedpassword@iscsi.example.org:3260/...