keyboard: Resetting a shortcut doesn't inform user if another shortcut is overwritten

When modifying a shortcut, gnome-control-center displays a binding that conflicts with the one being set, so the user can confirm they wish to override it.

Resetting a binding to its default doesn't do this. cc_keyboard_manager_reset_shortcut() checks for conflicts, but clears them without any prompt or notification. So if the user clicks the button to reset a shortcut to its default, they are not informed if another shortcut has been set to this same key.

Probably any method of changing a key binding, including this one, should prompt the user if they want to override another key.