fix-avsync frames jump back and forth when switching between videos

[AdventurerRussia]

mpv v0.35.1
https://www.youtube.com/watch?v=mtd6W4BEAbg
at 31 seconds, it is very clearly visible when I switch the video frames jump.

[dyphire]

In fact, this issue is very difficult to fix. because the working principle of this script is to fix A/V sync by use frame-step and frame-back. So sometimes you will see obvious frame jumps.

On the other hand, mpv has fixed the related issue: mpv-player/mpv#9591. This script is retained for the following reasons:
- mpv still stutters for about 1 second when switching audio tracks
- for users of older versions of mpv

Already fixed in upstream commit: mpv-player/mpv@940854c, no longer need this script.