The KK2 code has been in danger of out growing the memory available on the chosen chip used by the designer and as such the Instructions in the menu were dropped so more actual "working code" (read that as self level functionality and Camera Gimbal operation) could be implemented on the board.
The calibration routine still works, there is just no instruction screen or menu item.
You have a good point. If I were not familiar with KK2's past firmware versions, I would be completely lost as to the option or avaliable functionality of calibrating the ESCs throttle range through the KK2, since it is not actually mentioned in the menu.
If memory serves correctly the ESC calibration is a pass through on the KK2 board. In other words there is NO action taken or recorded by the board. It is all a transmitter, receiver, ESC thing. I always use several servo "Y" adapters cables stacked so I can calibrate all my esc's to my transmitter at 1 time. Then I plug everything up to the KK2 and away we go! Holding down buttons and powering up requires more hands than I have.
Repeat as required if changing Transmitter / Receiver's.