删除具有一个因子级别的变量，不包括NA

I need to drop the factor variables with one level (excluding NAs) in the nested dataset. The function below 'drop_fixed_factors' considers NA as one level while evaluating the number of factor levels. Help me fix that so that for A==Y, B has one level (A), not two (A, NA). I need to drop the factor variables with one le