lavc: remove img_get_alpha_info as it not available any more

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/libavcodec.v

@@ -3,7 +3,6 @@ LIBAVCODEC_$MAJOR {
                 audio_resample;
                 audio_resample_close;
                 #deprecated, remove after next bump
-                img_get_alpha_info;
                 dsputil_init;
                 ff_find_pix_fmt;
                 ff_framenum_to_drop_timecode;