Remove glance-centralized-cache job

This job was useful when the default value of the image_cache_driver
option was "sqlite". Nowadays, it defaults to "centralized_db", so the
job no longer serves any purpose.

Change-Id: I464609bde892e69064fefa060f43366470b9e17d

.zuul.yaml

@@ -247,18 +247,6 @@
             image:
               image_caching_enabled: True
 
-- job:
-    name: glance-centralized-cache
-    parent: tempest-integrated-storage-import
-    description: |
-      The regular job to test with glance centralized cache driver
-    vars:
-      devstack_local_conf:
-        post-config:
-          $GLANCE_API_CONF:
-            DEFAULT:
-              image_cache_driver: "centralized_db"
-
 - job:
     name: glance-grenade-centralized-cache
     parent: grenade-multinode
@@ -447,9 +435,6 @@
             irrelevant-files: *tempest-irrelevant-files
         - nova-ceph-multistore:
             irrelevant-files: *tempest-irrelevant-files
-        - glance-centralized-cache:
-            voting: false
-            irrelevant-files: *tempest-irrelevant-files
         - glance-grenade-centralized-cache:
             voting: false
             irrelevant-files: *tempest-irrelevant-files