Revert "[anchor-position] Make `anchor` attribute work for non-popovers"

This reverts commit 4abd0d04.

Reason for revert: Caused a lot of performance regressions.

Original change's description:
> [anchor-position] Make `anchor` attribute work for non-popovers
>
> This patch reimplements how we mark elements that are implicit anchors.
> Previously it was only marked for popover element's implicit anchors,
> and now it works for all implicit anchors. This is done by a new class
> AnchorElementObserver that observes all possible changes in implicit
> anchor and replaces the old PopoverAnchorObserver.
>
> This patch also reveals two existing issues:
> 1. Element::GetElementAttribute() may return non-null result even if
>    the element is out of tree scope. As fixing the bug is out of the
>    scope here, this patch just works around it. crbug.com/1425215 was
>    filed.
> 2. popover-anchor-idl-property.html has some bugs that made it pass
>    previously. This patch fixes those bugs.
>
>
> Fixed: 1417346
> Change-Id: I798977ab1ed7df0c528b7eb98c64fc04476e6106
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4338487


> Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
> Reviewed-by: Mason Freed <masonf@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1118933}

Bug: 1425610, 1425635, 1425650
Change-Id: I50a650f011e097c48c0efd110cf765d0df5835e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4353985


Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1119483}