avcodec/vc1dec: vc1_pred_b_mv() is not used for fields, simplify code

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/vc1dec: remove blocks_off use from vc1_pred_b_mv()

it should always be 0

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