Remove dumpstack() as it was unused

- I'm falling back on the "if someone wants it, they
  know where to find it" argument.
- This fixes an "unused function" warning on OS X. Although
  I previously marked it as "unused" that was really
  a bit too "sweep under the rug"-ish and I think my
  ultra-conservative imagined use case was a bit too
  imaginary.
Remove POLY_UNUSED macro

- This was only used in one place and that code is now
  gone so there's no more reason for this macro to stick
  around. It's use is not something that should be
  encouraged. So without a purpose, it needs to go.