refactorization: subroutine coupler_chksum removal

[mlee03]

This issue proposes to remove the subroutine coupler_chksum. In full coupler_main, the subroutine coupler_chksum is called before or at the end of the grouped calls to atmos_ice_land_chksum and ocean_chkum. In coupler_chksum, the checksums of tracers and the below select fields are computed twice and are written to stdout twice back to back.

Atm%t_box
Atm%z_bot
Atm%p_bot
Atm%u_bot
Atm%v_bot
Atm%p_urf
Atm%gust
Land%t_surf
Land%t_ca
Land%rough_mom
Land%rough_heat
Land%rough_scale
Ice%t_surf
Ice%rough_mom
Ice%rough_heat
Ice%rough_moist
Ice%ocean_fields

Unless it is useful to group the checksums of the above fields in stdout (for example, for easier analysis) coupler_chksum should be removed to avoid unnecessary redundancy.