lavfi: remove 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.

Also remove channel_layout check as it is now negotiated.

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
