Good question. All the ones you listed are required except for this set, which can be removed if you have generated your own language models:
OpenEars1.dic, OpenEars1.gram, OpenEars1.languagemodel. OpenEars2.gram. OpenEarsCorpus.txt
You can also remove COPYING, INSTALL and neatspeech_us1_README without affecting the app behavior, but I wouldn’t, since they contain important licensing information and are very small in size, and the user will never see the items inside the bundle.