You can delete the assignment by editing it, clicking the arrow on the right of the hotkey field, and selecting Clear from the menu.
You can change it by typing another key into the field.
What you're editing there is the actual Location field. It's the same thing you'll see if you go to Settings -> Customize, then right-click the Location field and choose Edit.
The hotkey is bound to the field itself, not to a command. (I think there might be a command which does the same thing but not certain off the top of my head.)