cmdutils: remove list_fmts(), simplify

The function was only used in opt_sample_fmt() for listing the sample
formats. Move list_fmts() functionality directly into
opt_sample_fmt().

Als fix the warning:
ffmpeg.c: In function ‘opt_audio_sample_fmt’:
ffmpeg.c:2877: warning: passing argument 1 of ‘list_fmts’ from incompatible pointer type
cmdutils.h:163: note: expected ‘void (*)(char *, int,  int)’ but argument is of type ‘char * (*)(char *, int,  enum AVSampleFormat)’

cmdutils.c

@@ -508,16 +508,6 @@ void show_license(void)
     );
 }
 
-void list_fmts(void (*get_fmt_string)(char *buf, int buf_size, int fmt), int nb_fmts)
-{
-    int i;
-    char fmt_str[128];
-    for (i=-1; i < nb_fmts; i++) {
-        get_fmt_string (fmt_str, sizeof(fmt_str), i);
-        fprintf(stdout, "%s\n", fmt_str);
-    }
-}
-
 void show_formats(void)
 {
     AVInputFormat *ifmt=NULL;