fftools/ffmpeg_mux_init: drop an always-false check

It cannot be true since 1959351aec. Effectively reverts 6a3833e141.

fftools/ffmpeg.h

@@ -674,7 +674,6 @@ extern HWDevice *filter_hw_device;
 extern unsigned nb_output_dumped;
 extern int main_return_code;
 
-extern int input_stream_potentially_available;
 extern int ignore_unknown_streams;
 extern int copy_unknown_streams;
 

fftools/ffmpeg_demux.c

@@ -1098,7 +1098,5 @@ int ifile_open(const OptionsContext *o, const char *filename)
         }
     }
 
-    input_stream_potentially_available = 1;
-
     return 0;
 }

fftools/ffmpeg_mux_init.c

@@ -1911,12 +1911,6 @@ int of_open(const OptionsContext *o, const char *filename)
         }
     }
 
-    if (!(oc->oformat->flags & AVFMT_NOSTREAMS) && !input_stream_potentially_available) {
-        av_log(NULL, AV_LOG_ERROR,
-               "No input streams but output needs an input stream\n");
-        exit_program(1);
-    }
-
     if (!(oc->oformat->flags & AVFMT_NOFILE)) {
         /* test if it already exists to avoid losing precious files */
         assert_file_overwrite(filename);

fftools/ffmpeg_opt.c

@@ -95,7 +95,6 @@ static int no_file_overwrite  = 0;
 #if FFMPEG_OPT_PSNR
 int do_psnr            = 0;
 #endif
-int input_stream_potentially_available = 0;
 int ignore_unknown_streams = 0;
 int copy_unknown_streams = 0;
 int recast_media = 0;
@@ -1114,8 +1113,6 @@ static int opt_filter_complex(void *optctx, const char *opt, const char *arg)
     if (!fg->graph_desc)
         return AVERROR(ENOMEM);
 
-    input_stream_potentially_available = 1;
-
     return 0;
 }
 
@@ -1130,8 +1127,6 @@ static int opt_filter_complex_script(void *optctx, const char *opt, const char *
     fg->index      = nb_filtergraphs - 1;
     fg->graph_desc = graph_desc;
 
-    input_stream_potentially_available = 1;
-
     return 0;
 }