Revert "Add a shim for #ppx directive for 4.02."

This reverts commit 5cda9ffb9935e84109e9b33d2ed6f4753a46cc29.

The patch was accepted into 4.02 after all:
https://github.com/ocaml/ocaml/commit/11bf27a9365166f933fe28972a7720fdfe58d1da