Drop DCTELEM typedef

It does not help as an abstraction and adds dsputil dependencies.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
dsputil: remove avg_no_rnd_pixels8.

This is never used.
dsputil: remove some never-assigned function pointers from the struct.
dsputil: remove 9/10 bits hpel functions.

These are never used.
dsputil: remove one array dimension from avg_no_rnd_pixels_tab.