Remove image-diff from zuul.d/projects.yaml

Instead have it define it's job in it's own .zuul.yaml file.

Change-Id: Iba18fd09860206106bf682520e69201ecb32c239
diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index 7205947..9ebaea8 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -173,11 +173,6 @@
       - non-core-plugin
 
 - project:
-    name: plugins/image-diff
-    templates:
-      - non-core-plugin
-
-- project:
     name: plugins/its-base
     templates:
       - non-core-plugin