Deprecated NewMapStruct

There was no reason for NewMapStruct, and it was dependent on github.com/fatih/structs. It's easy enough from someone to explicitly import the structs package and write two lines as opposed to one -

   sm, err := structs.Map(<some struct>)
   if err != nil {
      // handle error
   }
   m := mxj.Map(sm)

So don't make everyone vendor an extra package just for a function that most people aren't using anyway.