[sledge] Don't need staged_pps anymore

Summary:
Changing `pps` to `staged_pps` was needed for `import_ppx`, but it has
since been removed.

Reviewed By: jvillard

Differential Revision: D19973778

fbshipit-source-id: 5e2d83157

sledge/dune-common.in

@@ -36,7 +36,7 @@ let flags exe_or_lib deps =
       (ocamlc_flags (%s))
       (ocamlopt_flags (%s))
       (preprocess
-       (staged_pps
+       (pps
          ppx_compare
          ppx_custom_printf
          ppx_expect