mistral-dashboard is released as part of mistral

According to governance (and the first release in Ussuri),
the openstack/mistral-dashboard repository is released as part
of the mistral deliverable, so it does not need its extra deliverable
file.

Change-Id: Ie063c5fd3b6594afa6157d9a5764e3e45eaa7c4a

deliverables/ussuri/mistral-dashboard.yaml

@@ -1,7 +0,0 @@
----
-launchpad: mistral
-release-model: cycle-with-rc
-team: mistral
-type: horizon-plugin
-repository-settings:
-  openstack/mistral-dashboard: {}