Remove tripleo-image-elements from required project

As that does not exist anymore, hence results zuul config error, so
let's get rid of it.

Change-Id: Ibb41cef6c15085c591a76b204ceb0314523763dc

.zuul.yaml

@@ -56,7 +56,6 @@
     required-projects:
       - openstack/tempest
       - openstack/diskimage-builder
-      - openstack/tripleo-image-elements
       - openstack/trove
       - openstack/trove-dashboard
     vars:
@@ -458,7 +457,6 @@
     required-projects:
       - openstack/diskimage-builder
       - openstack/trove
-      - openstack/tripleo-image-elements
 
 - job:
     name: publish-trove-guest-image-ubuntu-bionic