Bug fix #3989 (fitting the ellipse)

Bug report and code sample: http://code.opencv.org/issues/3989
Currently, prior to the main calculations there is calculated an estimated centre of the ellipse. This step introduces some rounding errors which can lead in some cases to improper calculations of the parameters of the ellipse. Apparently, this initial calculation of the centre of the ellipse is not necessary and final calculation can be done only once when the main system of equations is solved.