[llvm] [bindings/OCaml] Remove unused dep on ounit2

Remove the dependency on ounit2 and the relevant lit code.  It seems
that ounit2 is not used at all and all OCaml binding tests pass without
it installed.

Thanks for Shiwei Weng and Josh Berdine for bringing this to
my attention.

Differential Revision: https://reviews.llvm.org/D119884