avfilter/formats: Schedule avfilter_make_format64_list() for removal

avfilter/formats: Schedule avfilter_make_format64_list() for removal

Despite its name, this function is not part of the public API, as
formats.h, the header containing its declaration, is a private header.
The formats API was once public API, but that changed long ago
(b74a1da49db5ebed51aceae6cacc2329288a92c1, the commit scheduling it to
become private, is from 2012). That avfilter_make_format64_list() was
forgotten is probably a result of the confusion resulting from the
libav-ffmpeg split.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>