centralize mongo schema handling logic

src/Adapters/Storage/Mongo/MongoSchemaCollection.js

@@ -126,12 +126,6 @@ function mongoSchemaFromFieldsAndClassNameAndCLP(fields, className, classLevelPe
     }
   }
 
-  // Legacy mongo adapter knows about the difference between password and _hashed_password.
-  // Future database adapters will only know about _hashed_password.
-  // Note: Parse Server will bring back password with injectDefaultSchema, so we don't need
-  // to add it here.
-  delete mongoObject._hashed_password;
-
   return mongoObject;
 }