No, there is no way to remove the amount on an Expense item. Whether it is a general expense or mileage.
Since you are not billing the Client for the mileage, what we would suggest is that you add yourself as a Client and put those mileage expenses under yourself. In the Purpose (notes) field, just enter for what client you were tracking your mileage for. Then at the end of the year, just run a report on the Expenditure code for your mileage. That way the mileage expenses will be grouped under you instead of your clients.
