mining: Remove comment and change fee calc to int.
mining: Remove comment and change fee calc to int.

Now that the memory pool minimum fee calculation code is also
calculating a more precise value instead of rounding up to the nearest
kilobyte boundary, the comment in NewBlockTemplate regarding this
behavior is no longer accurate.  Thus, this removes the comment.

Also, while here, change the calculation to use an int64 instead of
float since it matches the precision of the new calculation code used by
the memory pool and can also avoid the need for the slower floating
point math.

qchain/btcd,qchain/btcd