Remove useless OPUS_SET_EXPERT_FRAME_DURATION() on CELT

frame diration is only handled at the Opus layer. There's no change
in behaviour, just less puzzling code.

Thanks to Dmitriy <dmilvdv@gmail.com> for reporting this.

src/opus_encoder.c

@@ -2629,7 +2629,6 @@ int opus_encoder_ctl(OpusEncoder *st, int request, ...)
                goto bad_arg;
             }
             st->variable_duration = value;
-            celt_encoder_ctl(celt_enc, OPUS_SET_EXPERT_FRAME_DURATION(value));
         }
         break;
         case OPUS_GET_EXPERT_FRAME_DURATION_REQUEST: