When XBL is disabled, no code in dom/xbl will be built. Also, adds ifdefs
to remove any of the XBL related code elsewhere. There's definitely more
that can be done here, but I think it's better to wait to do the rest of
the cleanup when we actually remove the code.
Depends on D45612
Differential Revision: https://phabricator.services.mozilla.com/D45613
--HG--
extra : moz-landing-system : lando