Remove hash_behaviour=merge

I did some light testing and I don't think this is needed.  Switching the behavior on and off at least for edxapp with the deploy tag creates the same json configuration files, and I didn't think it was used anywhere else.  Having this  in has resulted in me being unable to replace the MixedModuleStore with just the MongoModuleStore in LMS since I can't get rid of the ```stores``` key which causes exceptions in the platform.