Remove the CN (non CENTURY) version of Carbon-Nitrogen soil biogeochemistry code and its testing

Remove the option to run the non-CENTURY and vertically resolved BGC code. This code isn't really being used, validated, or kept up to date. None of the current supported versions of the physics use it the default (clm4_5, clm5_0, nor clm5_1). There are also concerns for its scientific validity especially as compared to the full BGC version. The only advantage is it's simpler and faster, but because it isn't and hasn't been tuned in any of the current model physics versions it likely is just adding complexity.

This could be as simple as removing the following file from soilbiogeochemistry:

SoilBiogeochemDecompCascadeCNMod.F90

as well as the "CN" compsets and tests:

I2000Clm50Cn
I1850Clm45Cn

ERP_D_Ld9" grid="f19_g17" compset="I2000Clm50Cn" testmods="clm/drydepnomegan
ERP_P36x2_D_Ld5" grid="f10_f10_mg37" compset="I1850Clm45Cn" testmods="clm/default
ERP_P36x2_D_Ld5" grid="f10_f10_mg37" compset="I2000Clm50Cn" testmods="clm/default
SMS" grid="f19_g17" compset="I2000Clm50Cn" testmods="clm/default
SMS_D_Ld1" grid="f19_g17" compset="I1850Clm45Cn" testmods="clm/default
SMS_P48x1_D_Ld5" grid="f10_f10_mg37" compset="I2000Clm50Cn" testmods="clm/default

Doing that would be quite straightforward.