Revert "Clear last_focused_node in AX manager after load event is received"

This reverts commit 0a1a41b4ff6b816af7c51c1d75b9f2aaa776b8ad.

Reason for revert: Broke screen readers starting position https://crbug.com/1453789

Original change's description:
> Clear last_focused_node in AX manager after load event is received
>
> This ensures that a focus event is fired, even if a previous partial
> AX update contained the same focused node.
>
> Change-Id: I8708e592566087f3907b22f423fd4bf4f9c5a314
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4367844
> Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
> Commit-Queue: Stefan Zager <szager@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1127207}

Change-Id: Ie02f0ae1ed0f9441818e324490c029c6532655d3
Bug: 1453789
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4617714
Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Auto-Submit: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1160068}