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.h

@@ -161,8 +161,6 @@ void set_context_opts(void *ctx, void *opts_ctx, int flags, AVCodec *codec);
  */
 void print_error(const char *filename, int err);
 
-void list_fmts(void (*get_fmt_string)(char *buf, int buf_size, int fmt), int nb_fmts);
-
 /**
  * Print the program banner to stderr. The banner contents depend on the
  * current version of the repository and of the libav* libraries used by