Remove custom application operator

It appears that the custom application operator defined in quickCheck_util is already available in the [Pervasives module](http://caml.inria.fr/pub/docs/manual-ocaml/libref/Pervasives.html) since 4.01.

This just removes that definition and replaces the single use of it within quickCheck_gen.

The paucity of tests don't fill me with confidence that I haven't broken anything but the method signatures and implementations appear the same.