Fix toplevel w.r.t. ocaml.ppx.context attribute.

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15063 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
#6497: better fix, remove the context attribute in apply_rewriters.

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15064 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02