refactoring: remove exceptions from fk_constraint_def_new_from_tuple

fk_constraint_def_new_from_tuple is used in
on_replace_dd_fk_constraint therefore it has to be cleared from
exceptions. Now it doesn't throw any more. It means we also need
to clear from exceptions it's subsidiary function: decode_fk_links.
Their usages are updated. Some _xc functions, not needed any more,
are removed.

Part of #4247