cmdutils: remove error message from opt_default() in case of missing option

A similar error message is generated by the calling function, avoid
duplication and spurious error message occurring with negated boolean
options introduced in 8bb5680d, which can be reproduced for example with
the command:
ffmpeg -f lavfi -i testsrc=d=0.1 -nostdin -f null -

cmdutils.c

@@ -547,8 +547,6 @@ int opt_default(void *optctx, const char *opt, const char *arg)
 
     if (consumed)
         return 0;
-    av_log(NULL, AV_LOG_ERROR, "Could not find option '%s' in any of the FFmpeg subsystems "
-           "(codec, format, scaler, resampler contexts)\n", opt);
     return AVERROR_OPTION_NOT_FOUND;
 }