elements/config/ffmpeg.yml: do not build with sdl2

sdl2 is only used for the sdl output device (since we don't build ffplay).

This is a potentially breaking change, but:

• I don't think any app would use that directly (they ought to use gstreamer instead).
• The ABI checker didn't complain; this probably means the apps are supposed to check whether a device exists before using it.
• This will allow us not to rebuild gst-libav in the gnome runtimes (since sdl depends on gtk3 via ibus and fcitx), so in a sense, this is part of #314 (closed).