[nix] use standard ocp-indent nix package

nix/pkgs.nix

@@ -7,7 +7,6 @@ let
     headache = oself.callPackage ./headache.nix {};
     menhirLib = oself.callPackage ./menhirLib.nix {};
     mlmpfr = oself.callPackage ./mlmpfr.nix {};
-    ocp-indent = oself.callPackage ./ocp-indent.nix {};
     psmt2-frontend = oself.callPackage ./psmt2-frontend.nix {};
     why3 = oself.callPackage ./why3.nix {};