[library] Don't allocate library->raster_pool anymore

It's unused after the following commits:

  commit a6ff9bee1dae88971ef35e1a2ca39b2b76fe26b5
  [threadsafety] Allocate render pool for mono rasterizer on the stack

  commit 0610348976f376a931cea534ce5936136379b8c0
  [raster] Remove 5-level gray AA mode from monochrome rasterizer

The value of FT_RENDER_POOL_SIZE still serves the purpose it used to
serve, which is, to adjust the pool size.  But the pool is now
allocated on the stack on demand.