node.h: remove NODE_PRIVATE_RECV

* compile.c (private_recv_p): check by node type, instead of a
  magic number.
* node.h (NODE_PRIVATE_RECV), parse.y (attrset_gen): remove

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46366 b2dd03c8-39d4-4d8f-98ff-823fe69b080e