Is the February deal Cameron made with the EU already voided by the EU Referendum negative Vote? And if so, how did they implement such an mechanism in the contracts?
It is understood that, should the result of the referendum in the United Kingdom be for it to leave the European Union, the set of arrangements referred to in paragraph 2 above will cease to exist.
Yes. Cameron stated so himself on 2016-06-27:
The deal we negotiated at the European Council in February will now be discarded and a new negotiation to leave the EU will begin under a new Prime Minister.
AFAIK the deal was bound to a successful referendum.