Remove unused sum from particle_list_skeleton.h

[iljah]

GCC 16.1.1 gives:

In file included from user.cpp:25:
../../include/particle_list_skeleton.h: In instantiation of ”bool ParticleListSkeleton<SPECIES, PARTICLE>::updatePartitioning() [with SPECIES = Species; PARTICLE = Particle<double>]”:
../../include/particle_list_skeleton.h:150:40:   required from ”bool ParticleListSkeleton<SPECIES, PARTICLE>::accumulateBoundaryCells() [with SPECIES = Species; PARTICLE = Particle<double>]”
  150 |    if (sim->meshRepartitioned == true) updatePartitioning();
      |                                        ^~~~~~~~~~~~~~~~~~
../../include/particle_list_skeleton.h:143:6:   required from here
  143 | bool ParticleListSkeleton<SPECIES,PARTICLE>::accumulateBoundaryCells() {
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../include/particle_list_skeleton.h:937:11: warning: muuttuja ”sum” asetettu mutta käyttämätön [-Wunused-but-set-variable=]
  937 |    size_t sum = 0;
      |           ^~~