Don't generate dead code

[mjambon]

This is a follow-up to #210. We now extract fewer rules for token() constructs, but it looks like the generated files Boilerplate.ml and CST.ml still contain code for those as if they had been extracted and given a rule name. I'll look into this. It would be best to not generate code that's not used. OCaml can report it depending on the flags passed to the compiler; that's how I noticed but too late.