audio: remove unnecessary code

It's true that the decoder can successfully decode, but return no data
(for various reasons). We don't need to handle this specially, though.
We just let the decoder decode some more data. This doesn't increase the
danger of an endless loop either, because audio_decode() already calls
this function until enough is decoded.

audio/decode/dec_audio.c

@@ -267,9 +267,6 @@ static int filter_n_bytes(struct dec_audio *da, struct mp_audio_buffer *outbuf,
         error = da->ad_driver->decode_packet(da);
         if (error < 0)
             break;
-        // No progress means the decoder is buffering input data.
-        if (!da->decoded.samples)
-            break;
     }
 
     // Filter