aout: remove unused "equalizer" object variable

This is no longer set anywhere, since equalizer-preset superseded it.
aout: remove the bands callback (fixes #9350)

playlist_EnableAudioFilter() can be used to turn the equalizer on or off.
The GUI already does so.
aout: fix assertion on error path from transcode
lib: enable/disable the equalizer as needed, simplify
aout: move input restart on filter change from playlist code

So that it works for LibVLC too.
aout: do not restart input if filter string is unchanged

This avoids glitching when changing the equalizer bands from LibVLC.