IMPORTANT NOTE: This method can be used ONLY if you accidentally added a C++ class or to just compile a plugin. If you have classes created in C++ and using them then obviously you cant convert it back to Blueprints.
This question was actually asked on UE4 AnswerHUB and it got me thinking is there way to do it? If you accidentally added a C++ class can you convert the project back to Blueprints? Or if you added a C++ class just to compile a plugin can you convert the project back to Blueprint only project?
More info: http://wp.me/p2fInN-mK