ocamlPackages: remove pre 4.03 ppx_* janestreet packages

These packages have been failing to evaluate for some time now for two
reasons:

* ocamlmod doesn't evaluate for OCaml < 4.04 which is fixed in NixOS#113771
* We have no ppx_core version for OCaml < 4.03 which is a fundamental
  issue.

Since this situation has been going on for some time now, I don't really
see the need to figure out how to fix these legacy versions of some
janestreet packages. There is also additional motivation:

* These packages reference the janeStreet set directly which I'd like
  to deprecate, as it spells trouble for the overriding possibilities
  introduced in NixOS#113696.
* These packages are patched in awkwardly into the janeStreet set
  without forming one themselves. By removing them we can move closer to
  having all janestreet packages managed uniformly in their own
  janestreet sets.