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!