dsputil: make square put/avg_pixels functions local to h264qpel.

Put a copy of the 8bit functions only in dsputil, where they are used
for some other things (e.g. mpeg4qpel, mspel, cavsqpel). We could perhaps
also try to share specifically the 8bit functions from h264qpel between
it and the others, but that will be slightly more complicated. H264qpel
already had these functions, so we can simply remove the duplicates.

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