lavf: remove duplicate assignment in avformat_alloc_context.

AVClass is already initialized in  avformat_get_context_defaults.

libavformat/options.c

@@ -86,6 +86,5 @@ AVFormatContext *avformat_alloc_context(void)
     ic = av_malloc(sizeof(AVFormatContext));
     if (!ic) return ic;
     avformat_get_context_defaults(ic);
-    ic->av_class = &av_format_context_class;
     return ic;
 }