Remove offer_nice_chunk(), PL_nice_chunk and PL_nice_chunk_size.

Remove offer_nice_chunk(), PL_nice_chunk and PL_nice_chunk_size.

These provided a non-public API for the hash and array code to donate free
memory direct to the SV head allocation routines, instead of returning it
to the malloc system with free().

I assume that on some older mallocs this could offer significant benefits.
However, my benchmarking on a modern malloc couldn't detect any significant
effect (positive or negative) on removing the code. Its (continued) presence,
however, has downsides

a: slightly more code complexity
b: slightly larger interpreter structure
c: in the steady state, if net creation of SVs is zero, 1 chunk of allocated
   but unused memory will exist (per thread)

So I think it best to remove it.