closes#5854
Cancelling task runs doesn't make any sense.
- It breaks sequential task execution - later tasks might depend on state from earlier tasks
- It doesn't actually cancel the task - at best, it prevents it from running, but cannot interrupt it (though interrupting a task does not make sense either)
We don't use this "feature" in the core anymore since 22b5e5db5e, as this was causing unexpected behaviour for plugins anyway, along with the occasional shutdown crash due to inconsistent worker states.
a74ab756bd