avcodec: Remove deprecated AVCodecContext.coded_frame

Deprecated in 40cf1bbacc6220a0aa6bed5c331871d43f9ce370.
(The currently disabled filter vf_mcdeint and vf_uspp were users of
this field; they have not been changed, so that whoever wants to fix
them can see the state of these filters when they were disabled.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Signed-off-by: James Almer <jamrial@gmail.com>