To remove a dependency between two tasks on the diagram view, draw a line from the source to the target task, just like you did when adding it. The line you draw will turn orange to indicate that a dependency between these tasks exists and can be removed. Just lift your finger to delete it.
Alternatively, you can choose to delete all dependencies of the target task by tapping on it. Next, tap on the dependency icon on the right side of the task sheet. Confirm to delete all dependencies of this task.