Remove ReadAnythingAppModel::AddTree().

Everything this does can be done inline in the lone caller. Simplify
while we're at it.

This also eliminates a test that just tests this method and tree
destruction; the test immediately following tests the same
functionality, using the AccessibilityEventReceived() call that
production code actually uses.

Bug: 364987728
Change-Id: I8e837dea4827b7bf2581239572162dc4b8fcd1a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6376835
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/main@{#1435744}