huffyuvdec: remove somewhat deprecated code

The old code was reserving the 0xFFFF entry to represent an inexisting
entry/codeword. These entries are now detected through their length
being <= 0. As this entry is often used for the residuals (-1,-1), which
should be among the most frequent, it is particularly important to not
reserve it.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>