Remove redundant sea ice enthalpy flux term

[cbegeman]

The enthalpy flux associated with seaIceFreshWaterFlux is already included in seaIceHeatFlux which derives from fhocn in Icepack. Thus, it was incorrect to apply both seaIceHeatFlux and seaIceTemperatureFlux in MPAS-Ocean, as seaIceTemperatureFlux provides the thickness of seaIceFreshWaterFlux at the freezing point.

Here, we set seaIceTemperatureFlux to 0 always. The variable seaIceTemperatureFlux could be removed entirely if desired. I retain it here for clarity.

[cbegeman]

Closing until E3SM-Ocean-Discussion#135 is approved