avformat: remove pts hack on recent libavformat

The field AVStream.cur_dts has been removed in libavformat 59.2.103.

Refs vlc4/c2ba623f
Refs ffmpeg/591b88e6787c4e678237f02a50421d101abd25c2
8 jobs for 3.0.x in 19 minutes and 41 seconds (queued for 4 seconds)