Remove a bit of dead code in InertSubtreesChanged (for <dialog>)

Because of how Document::ClearAXObjectCache() is implemented,
Document::ExistingAXObjectCache() will always return nullptr
immediately after, so the interesting code was unreachable.

The code is from https://codereview.chromium.org/70213002.

Bug: 787867
Change-Id: I08c8eed89f92749f7f12162e9d41ee17b4d172c9
Reviewed-on: https://chromium-review.googlesource.com/787910
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519169}
1 file changed