Remove redundant argument in think()

We don't need to pass side_to_move because we can get
it directly from the position object.

Note that in benchmark we always used to pass '0' and
it was a bug, but with no effect because was used only
in time[] and increment[], set always to 0 for both
colors.

Also additional small cleanup while there.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>