Fix buggy pref for disabling MS Exchange autoconfig method.

Before the upstream work that made the autoconfig methods happen in
parallel this was working, but when I adapted it to that work I seem
to have made a mistake or misunderstood something. In fact, the error
propagation now happens in a way that makes it very unsuitable to have
this check this deep down in the call stack. The check should happen
earlier.

So let's just not initiate this autoconfig method at all when
disabled, with the added benefit of it not showing up as an
immediately failed method in the list of attempts. This makes so much
more sense!

While we're at it, let's do the same for when the MX method is
disabled since there is no reason for it to be listed as a failure
when disabled either.