remove some dead code and prune argument lists accordingly

src/USER-MEAMC/meam.h

src/USER-MEAMC/meam_dens_final.cpp

src/USER-MEAMC/meam_dens_init.cpp

src/USER-MEAMC/meam_force.cpp

src/USER-MEAMC/meam_setup_done.cpp

src/USER-MEAMC/pair_meamc.cpp