simplify HistogramCombineEntropyBin()
simplify HistogramCombineEntropyBin()

We only perform a single pass, and swap the final histograms
into the beginning of the array as we go. Therefore, they are
already at the correct place at the end of the pass.
-> HistogramCompactBins() is removed, we just truncate the array.

output is bitwise the same.

Change-Id: I9508c96dda0f8903c927a71b06af4e6490c3249c

webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp