Energy2D: Rm backing_scale parameter from heatmap

[#52558183]

... to avoid confusion. We can safely rely on default browser
behavior, as now .getImageData(0, 0, W, H) always returns array with
W * H elements. One year ago we had to detect if we are not dealing
with Retina displays and do some hacks to ensure that we operate on
100 x 100 array.