Remove a reference to the hydro tasks in the scheduler.

This is a quick one-line change but just wanted to check with you that this is OK.

The goal is to remove all explicit reference to (hydro) task names in the scheduler to keep it generic and re-usable as-is for the gizmo scheme. Will do a similar clean-up for the gravity bit later on.

Re-assign to me if you are happy with the change.