Remove unnecessary call to png_set_sBit when writing png files

Remove unnecessary call to png_set_sBit when writing png files

According to libpng documentation - PNG files reduce possible bit depths
to 1, 2, 4, 8, and 16. If data is of another bit depth, it's possible to
write an sBIT chunk into the file so that decoders can recover the
original data if desired. Since we hardcode depth to 8 there is no need
to call png_set_sBit

Change-Id: I8e3941675019b920051775128ff4cf2bf1ca7c4a
Reviewed-by: aavit <eirik.aavitsland@digia.com>