Fix warning: unused variable in diffusion.hpp

[mloskot]

Description

@simmplecoder Could you confirm these were just unused and not meant to be used but overlooked?

../../boost/gil/image_processing/diffusion.hpp:160:14: warning: unused variable ‘first’ [-Wunused-variable]
  160 |         auto first = stencil.begin();
      |              ^~~~~
../../boost/gil/image_processing/diffusion.hpp:161:14: warning: unused variable ‘last’ [-Wunused-variable]
  161 |         auto last = stencil.end();
      |              ^~~~

Tasklist

• Review and approve

[simmplecoder]

It seems like I wanted to add a warning or error if the stencil is not of the correct size (it should be typical 9 pixel window), but backtracked on that because the function is usually called in a loop and rarely used on its own. If the error message is not needed, then it is safe to merge this. The reduction looks correct (it is 5 connectivity, diagonals and the center).