Philippe 09/01/2020: remove unused dummy arguments for gradient_*.f90 functions

# Conflicts:
#	src/MNH/gradient_m.f90
#	src/MNH/gradient_u.f90
#	src/MNH/gradient_w.f90
#	src/MNH/prandtl.f90
#	src/MNH/turb.f90
#	src/MNH/turb_hor_dyn_corr.f90
#	src/MNH/turb_hor_sv_corr.f90
#	src/MNH/turb_hor_thermo_corr.f90
#	src/MNH/turb_hor_uv.f90
#	src/MNH/turb_hor_uw.f90
#	src/MNH/turb_hor_vw.f90
#	src/MNH/turb_ver_dyn_flux.f90
#	src/MNH/turb_ver_thermo_flux.f90