avfilter/formats: Remove unused functions

avfilter/formats: Remove unused functions

This commit removes ff_parse_sample_format(), ff_parse_time_base() and
ff_query_formats_all_layouts() from libavfilter/formats.c. All of these
functions were completely unused. ff_parse_time_base() has not been used
at all since it had been added in 3448404a707b6e236a2ffa7b0453b3300de41b7b;
the last caller of ff_parse_sample_format has been removed in commit
d1c49bcae9b7fd41df5c6804ac7f6a5c271a7c2e. And the one and only caller of
ff_query_formats_all_layouts() (the asyncts filter) has been removed in
commit a8fe8d6b4a35c95aa94fccde5f001041278d197c.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>