Type of operation is related to a specific implementation
of a task. Then, it should rather be access with a virtual
method in tasks::task_manager::task::impl than be
its attribute.
Closes#12326
* github.com:scylladb/scylladb:
api: delete unused type parameter from task_manager_test api
tasks: repair: api: remove type attribute from task_manager::task::status
tasks: add type() method to task_manager::task::impl
repair: add reason attribute to repair_task