Add dequeue event handler to the check pipeline

Change-Id: I949c667a8161da1cdcf3471753aa1867759e09f8
diff --git a/zuul.d/pipelines.yaml b/zuul.d/pipelines.yaml
index 8b21bdd..3ed9680 100644
--- a/zuul.d/pipelines.yaml
+++ b/zuul.d/pipelines.yaml
@@ -30,6 +30,12 @@
           scheme: 'zuul-check'
           state: NOT_RELEVANT
           message: 'Project has no jobs configured'
+    dequeue:
+      gerrit:
+        checks-api:
+          scheme: 'zuul-check'
+          state: NOT_RELEVANT
+          message: 'Change has been dequeued'
     success:
       gerrit:
         comment: False