To remove sub-task from Scheduled Issue, go to the Scheduled Issue details page, then click on the Delete icon next to the Sub-Task:
%20-%20step%201.png?version=1&modificationDate=1544096214837&cacheVersion=1&api=v2 "The Scheduler > Delete Scheduled Issue Sub-Task [5.x] > Delete Scheduled Issue Sub-Task (5.x) - step 1.png")