remove dead code in various neighbor list related source files

src/USER-DPD/fix_shardlow.cpp

src/USER-DPD/npair_half_bin_newton_ssa.cpp

src/USER-OMP/npair_half_size_bin_newton_omp.cpp

src/USER-OMP/npair_half_size_nsq_newtoff_omp.cpp

src/USER-OMP/npair_half_size_nsq_newton_omp.cpp

src/npair_half_size_bin_newtoff.cpp

src/npair_half_size_bin_newton.cpp

src/npair_half_size_bin_newton_tri.cpp

src/npair_half_size_nsq_newtoff.cpp

src/npair_half_size_nsq_newton.cpp

src/npair_skip_size.cpp

src/npair_skip_size_off2on.cpp

src/npair_skip_size_off2on_oneside.cpp