finish zsalinity deprecation

[eclare108213]

There are still a few references to zsalinity in the icepack driver and doc, which could be cleaned up:

icepack/configuration/driver/icedrv_init_column.F90:          solve_zsal = .false.   ! deprecated with zsalinity
icepack/configuration/driver/icedrv_init_column.F90:          grid_oS, l_skS    ! deprecated with zsalinity
icepack/configuration/driver/icedrv_init_column.F90:        restore_bgc, scale_bgc, solve_zsal, &
icepack/configuration/driver/icedrv_init_column.F90:      ! zsalinity and brine
icepack/configuration/driver/icedrv_init_column.F90:      if (solve_zsal) then
icepack/configuration/driver/icedrv_init_column.F90:         call icedrv_system_abort(string=subname//'ERROR: zsalinity is deprecated ', &
icepack/doc/source/user_guide/ug_case_settings.rst:..   "``solve_zsal``", "logical", "update salinity tracer profile", "``.false.``"
icepack/doc/source/user_guide/ug_case_settings.rst:..   "TRZS", "0,1", "zsalinity tracer, needs TRBRI=1", "0"

I only searched for zsal so some related variables like grid_oS and l_skS might be in other places.