libre-testing: remove ffmpeg
    
    The package seems not to be maintained, and to use an old 4.2 version
    of ffmpeg and we now use ffmpeg 4.4 from extra.
    
    That package was added to remove support for CUDA which is nonfree.
    
    The CUDA support in ffmpeg seems to have moved to LLVM (with the
    --enable-cuda-llvm option) and the llvm package of Arch Linux seems to
    be built with some CUDA support but it doesn't depends on any CUDA
    package (else it would depend on cudnn or another pakcage with 'cuda'
    in its name).
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>