lavfi: deprecate default config_props() callback and refactor avfilter_config_links()

lavfi: deprecate default config_props() callback and refactor avfilter_config_links()

Link properties have to be checked after config_props() is called to
make sure everything is sane, so the default config_props() for output
links was redundant.

Remove now empty defaults.c

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