Tests: don't wait for completion while trying to get completed task

Nodes are reusing task ids after restart. So in some rare circumstances
the same task id might be assigned to the reindexing task stored by the
old cluster and the new task that is trying to retrieve the task
results. As a result, the get task request can timeout waiting on
itself. Since we already waited for the task to finish before restarting
the cluster, waiting for the task here doesn't make any sense to start
with.

Fixes elastic#28732

Author: imotov

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/upgraded_cluster/10_basic.yml

@@ -141,7 +141,6 @@
 
   - do:
       tasks.get:
-        wait_for_completion: true
         task_id: $task_id
 
   - is_false: node_failures