symmetric relations: replace bogus rql2sql translation by a hook The hook ensures X r Y => Y r X iff r is symmetric. The rql-no-hook data importer receives a small amendment but note that: * there exist no test for it * its actual semantics are undefined Hence we cannot prove this hunk breaks nothing, because we cannot prove anything. Closes #3259713.

add_attribute('CWAttribute', 'extra_props')