examples/filtering_audio,video: do not call avcodec_register_all()

It is implied by av_register_all(). Simplify.

doc/examples/filtering_audio.c

@@ -210,7 +210,6 @@ int main(int argc, char **argv)
         exit(1);
     }
 
-    avcodec_register_all();
     av_register_all();
     avfilter_register_all();
 

doc/examples/filtering_video.c

@@ -200,7 +200,6 @@ int main(int argc, char **argv)
         exit(1);
     }
 
-    avcodec_register_all();
     av_register_all();
     avfilter_register_all();