Remove "prefix" from "Schedule Jobs" UI

When editing a job, there are three unlabeled fields used for designating an API call: "prefix", "entity", and "action." This is different from the normal API convention of using only "entity" and "action" and implies that the scheduler does some adhoc string concatenation ($function_name = $prefix + $entity + $action) and bypasses the normal civicrm_api(). For an administrator who wants to add a new job, this field is confusing/mysterious.