media-video/mpv: drop libav support in 9999

media-video/mpv: drop libav support in 9999

Rationale:
1. Fails to build against libav and mpv upstream doesn't care:
   https://github.com/mpv-player/mpv/issues/6007
2. Libav unmaintained in Gentoo:
   - security bumps missing for months: 651220
   - security bugs untouched for months: 635520, 635522, 635524, 639770
   - sources unfetchable for months: 641678
   - transition to libav-12 incomplete for more than a year: 603726

Let's stop pretending libav support is fine, when it's clearly not.
If you want mpv with libav back, please fix the above issues first.

Package-Manager: Portage-2.3.43, Repoman-2.3.10