a11y: remove unneeded copy from AXTree::AXUpdateState

Requires removing logic to add old/new focus ndoes to pending node
data. This doesn't appear to be needed anymore since all managers of
trees already account for focus changes more robustly (focus can change
from one tree to another). Thus, this block seems already incomplete.

Removing allows us to make use of the already populated pending tree
data on AXUpdateState. That is already a raw ref. This saves us from
copying the entirety of the AXTreeUpdate/AXNodeData for every single
update.

Change-Id: If0164a0fa211ebe540a824e546615ea977f28423
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6613004
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1468364}