If you want to remove assignments from multiple shifts, there is a quick and easy way to do this.
Select shifts
First, select all shifts from which you want to remove the assignment.
Our hotkeys make this step even quicker:
Select multiple shifts: cmd + alt + mouse click on the desired shifts
Select the whole day: cmd + alt + mouse click on the desired day
Select all layers: cmd + alt + A
Remove assignments
To remove the assignments, with the shifts still selected, click on the x in the circle on the left above the duty roster.
The selected shifts are now unassigned.