remove unnecessary field
remove unnecessary field

darvik80/msgp,eliothedeman/msgp,appneta/msgp,ebfe/msgp,jfbus/msgp,skatsuta/msgp,dockbiz/msgp,segmentio/msgp,tinylib/msgp,DrGo/msgp,tinylib/msgp,kgrz/msgp,appneta/msgp,mengzhuo/msgp