avcodec/mpegvideo_dec: Remove commented-out cruft

This piece of code has been added in an already commented-out state
in commit 158c7f059c3906b0116ae73a7669a3bb7bc570bb. It certainly
doesn't make sense now (if ever) because new_picture_ptr it used
has been removed in 6571e41dcdee4655237bca0a3277df8976167aee
(and new_picture is only used for encoding).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
