Older versions do not have "override" parameter in add_role_to_domain()
function signature. Luckily the override is _not_ required when
overidding the built-in standard domain roles for the first time, so we
just drop the paramter.
Tested with Sphinx 1.6.7 (does not have override) and Sphinx 4.5.0
(does have override).
Fixes: #3294
Related: !6086
02f5e9c505