Remove obsolete #ifdef OPUS_BUILD.

We always define OPUS_BUILD, so this code has no effect.
Likewise there is no reason for it to be conditional.
According to Jean-Marc it's left over from when we had
an alternate CELT_BUILD define.

celt/celt_decoder.c

@@ -1080,7 +1080,6 @@ int opus_custom_decoder_ctl(CELTDecoder * OPUS_RESTRICT st, int request, ...)
          *value = st->postfilter_period;
       }
       break;
-#ifdef OPUS_BUILD
       case CELT_GET_MODE_REQUEST:
       {
          const CELTMode ** value = va_arg(ap, const CELTMode**);
@@ -1103,7 +1102,6 @@ int opus_custom_decoder_ctl(CELTDecoder * OPUS_RESTRICT st, int request, ...)
          *value=st->rng;
       }
       break;
-#endif
       default:
          goto bad_request;
    }