af_drc: remove

Remove low quality drc filter. Anyone whishing to have dynamic range
compression should use the much more powerful acompressor ffmpeg filter:

    mpv --af=lavfi=[acompressor] INPUT

Or with parameters:

    mpv --af=lavfi=[acompressor=threshold=-25dB:ratio=3:makeup=8dB] INPUT

Refer to https://ffmpeg.org/ffmpeg-filters.html#acompressor for a full
list of supported parameters.

Signed-off-by: wm4 <wm4@nowhere>