Thunderbird: drop obsolete patch.

It's unclear to me why we needed this patch:

https://bugzilla.mozilla.org/show_bug.cgi?id=1680483

We definitely needed it with Thunderbird 68, but possibly because of one of our other patches, as I haven't been able to reproduce in (unpatched) Thunderbird 68.x. A bit of a mystery, but I don't think me trying to understand exactly what happened is a good use of my time: everything works fine without this patch now, let's just roll with that.

That patch also had a nice side-effect of not marking the MS Exchange autoconfig method (and the MX method) as failures when disabled that we'll lose now. But it's not worth the efforts needed to either upstream or maintain on our side.