kibana: Remove 6.x migration from upgrade

This was required in Train and Ussuri for the 5.x to 6.x upgrade, but is
no longer necessary.

Change-Id: I8a08f1c411c243c4d1cb6efd0e82de67a5151197
(cherry picked from commit 7020f96)

Author: markgoddard

ansible/roles/kibana/tasks/migrate-kibana-index.yml

@@ -3,7 +3,5 @@
 
 - import_tasks: check-containers.yml
 
-- import_tasks: migrate-kibana-index.yml
-
 - name: Flush handlers
   meta: flush_handlers