Don't warn on synchronous XHRs.

Note that there are plenty of other issues with synchronous XHRs in Gecko/Firefox compared to all other major browsers (including IE), see: https://bugzilla.mozilla.org/show_bug.cgi?id=697151

That said, those are fixable on wine's mshtml side, so using Gecko's existing sync XHR implementation should work; and we have to handle some of them there, anyway, such as for message events for example, so might as well just handle them all on wine side. The prerequisite that's needed for that, which is an event loop/message pump, is already working fine in wine-gecko, even though it's "deprecated".