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

Also 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)’

Signed-off-by: Anton Khirnov <anton@khirnov.net>