The x86 optimized dct_quantize only calculates the last nonzero
coefficient correctly if the zigzag scan order is used. For the
alternate scan order, this value is incorrect.
To work around this, the dct_unquantize functions process the entire
block if the alternate scan order is used.
But a second workaround (bb198e198a) was added that recalculates the
last nonzero coefficient after dct_quantize is called if the alternate
scan order is used.
This commit removes the first workaround, which became redundant.
