Remove show_image_direct_url for external ceph

Currently glance v2 doesn't allow to specify custom
locations for image by default, remove it for external ceph too.

Related to https://review.openstack.org/#/c/518864/
Related to https://review.openstack.org/#/c/279630/2

Change-Id: I24fbf22bd158a38db4b98d74a951dfc01193fbc6

doc/source/reference/external-ceph-guide.rst

@@ -57,9 +57,6 @@ Step 1 is done by using Kolla's INI merge mechanism: Create a file in
 
 ::
 
-  [DEFAULT]
-  show_image_direct_url = True
-
   [glance_store]
   stores = rbd
   default_store = rbd