duplicate definition of sbcssm variables in field_def_nemo files

sbcssm variables are defined twice, in field_def_nemo-ice.xml and in field_def_nemo-oce.xml.

1. I am not sure it is needed or not (for example when running SAS?)
2. However there is a clear issue with the grid_ref definition:

• in field_def_nemo-oce.xml, we have

      <field id="sst_m"    unit="degC" grid_ref="grid_T_2D" />
      <field id="sss_m"    unit="psu"  grid_ref="grid_T_2D" />
      <field id="ssu_m"    unit="m/s"  grid_ref="grid_T_2D" />
      <field id="ssv_m"    unit="m/s"  grid_ref="grid_T_2D" />
      <field id="ssh_m"    unit="m"    grid_ref="grid_T_2D" />
      <field id="e3t_m"    unit="m"    grid_ref="grid_T_2D" />
      <field id="frq_m"    unit="-"    grid_ref="grid_T_2D" />

• whereas in field_def_nemo-ice.xml, we have

    <field_group id="SBC_2D" grid_ref="grid_T_2D_inner" >
...
      <!-- sbcssm variables -->
      <field id="sst_m"    unit="degC" />
      <field id="sss_m"    unit="psu"  />
      <field id="ssu_m"    unit="m/s"  />
      <field id="ssv_m"    unit="m/s"  />
      <field id="ssh_m"    unit="m"    />
      <field id="e3t_m"    unit="m"    />
      <field id="frq_m"    unit="-"    />

in sbc_oce.F90, we have

      ALLOCATE( fr_i(jpi,jpj) ,     &
         &      ssu_m  (jpi,jpj) , sst_m(jpi,jpj) , frq_m(jpi,jpj) ,      &
         &      ssv_m  (jpi,jpj) , sss_m(jpi,jpj) , ssh_m(jpi,jpj) , e3t_m(jpi,jpj) , STAT=ierr(4) )

Which suggests that we should get rid of the field_def_nemo-ice.xml definition...