At no point does MOM6 code actually set arrays passed to the post_data() to
have a missing value.  Instead a missing value is set in output files entirely
by masking.  This commit eliminates the logic that would (inaccurately) try to
reset fields that seem to match rescaled missing values to the output missing
value.  The previous code was inaccurate, in that a rescaled field could have
taken on the unscaled missing value as a valid data point and still have been
incorrectly marked is missing, although the odds of this happening are
exceptionally small and it would only be cases with dimensional rescaling where
this could have applied.  For 2-d diagnostics, this commit eliminates a
duplicative array syntax math expression that did exactly what the code now
does. All solutions are identical, and because the missing value was not being
explicitly it is unlikely that any diagnostics will change.