avfilter/transform: Stop exporting internal functions

avfilter_transform, avfilter_(add|sub|mult)_matrix are not part of the
public API (transform.h is not a public header), yet they are currently
exported because of their name. This commit changes this:
avfilter_transform is renamed to ff_affine_transform; the other
functions are just removed as they have never been used at all.

Found-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>