Remove image_format config section

If the reference to the external-ceph-guide file configuration,
glance only supports raw image format.
I test can work normally of qcow2 image.
If enable ceph in kolla,the config section is not config.
see:
https://github.com/openstack/kolla-ansible/blob/master/ansible/roles/glance/templates/glance-api.conf.j2

Change-Id: Ie42be099692049b9f07e869e833b62000cff9dcc

doc/external-ceph-guide.rst

@@ -67,10 +67,6 @@ Step 1 is done by using Kolla's INI merge mechanism: Create a file in
   rbd_store_user = glance
   rbd_store_ceph_conf = /etc/ceph/ceph.conf
 
-  [image_format]
-  container_formats = bare
-  disk_formats = raw
-
 Now put ceph.conf and the keyring file (name depends on the username created in
 Ceph) into the same directory, for example: