If you created a custom task that you no longer need to track, you can delete the task so it no longer shows in the list of tasks when you add a new time entry
To delete a task:
Tap the 3 dots in the top left corner
Select Time & Tasks
Tap the Gear icon
Tap Tasks
Tap the Task you want to delete
Tap Delete Task
If you want to restore a deleted task:
Tap the Gear icon in the Time and Tasks section
Tap the 3 horizontal bars in the top right
Tap Archived
Tap on the task you want to restore, then tap Restore Task