CI: stop setting ceph_nova_user

Once I6aa8db2214e07906f1f3e035411fc80ba911a274 has been merged and
backported, the default value of ceph_nova_user (cinder) should work.

Change-Id: I9769c4ee26393f8458537ad2ad543213cf8f57b0
(cherry picked from commit bd3ad90)

Author: markgoddard

tests/templates/globals-default.j2

@@ -121,7 +121,6 @@ enable_cinder: "yes"
 glance_backend_ceph: "yes"
 cinder_backend_ceph: "yes"
 nova_backend_ceph: "yes"
-ceph_nova_user: "cinder"
 
 # TODO(yoctozepto): Remove this in the Xena cycle.
 # cephadm doesn't support chrony in a container (checks for chrony.service)