avfilter/fifo: Remove unused functions and headers

avfilter/fifo: Remove unused functions and headers

The functions were forgotten in 03c8fe49ea3f2a2444607e541dff15a1ccd7f0c2;
removing them also means that the avassert.h and samplefmt.h headers are
no longer used any more, so they have been removed, too.

Moreover, video.h is unused since b077d8d9082d057d4c7abd9e0b1a98f9651cfaa8
and channel_layout.h is since fdd9663781e3ebc8ebed0704607abd174095a905.
Both headers have therefore been removed, too.

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