You'll need to remove/delete the Entry Source (data extension) in order to remove the schedule. But copying the Journey when you get the "The schedule for this entry source exists in the past. If you create a copy of this journey, you must create a new entry source definition." error will do the same thing.
If you have the location of your Entry Event data extension you can go about it in two ways.
(1) Copy the journey without removing the data extension, ignore the warning message,it will allow you to make the copy. The copy will not have an entry event when created.
(2) Delete the Entry event (if you hover over the Entry Event circle you'll see a small trash icon to delete). Then you can copy the Journey without receiving the message but the end result will be the same as step 1. WARNING: Only delete the entry event if the Journey you are using has been archived and not used as future drafts will not be reliable when deleting a DE and adding it back.
When making a copy make sure all your activities are validated when you reconnect the entry event and schedule.