Remove NOP
It do not exists anymore because it was replaced by CALL_ME_LATER
/ REQUESTED_CALL
if I understand correctly.
So we need to remove it from the doc.
Also interestingly we have kept the old protocol letter mapping (not used anymore) that contains also NOP: https://gitlab.inria.fr/batsim/batsim/blob/master/src/network.hpp
We need to remove it too!