In order to hide a column in normal mode of RadGrid you could just set its Visible or Display property to false. There is no need to modify it in the code-behind.
Note that the columns are rendered differently depending on the used property. If Visible is set to false the values from that column are not persisted in the ViewState. If you need to use values from the hidden column you should use the Display property.
On a side note - if you need the reverse option (the column to be visible in normal mode and hidden in edit mode) you could set the ReadOnly property to true.
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