Revert "Implement new dialog shadow focus behavior"

This reverts commit afeccd1f5b2dd72595abe55bc9ef093f11b495f1.

Reason for revert: dialog-focus-shadow.html is failing on a couple of mac builders:

https://ci.chromium.org/ui/p/chromium/builders/ci/Mac10.13%20Tests/57992/overview
https://ci.chromium.org/ui/p/chromium/builders/ci/Mac10.14%20Tests/28597/overview

Original change's description:
> Implement new dialog shadow focus behavior
>
> This implements the new dialog initial focus behavior specified in these
> changes:
> whatwg/html#7079
> whatwg/html#7284
> whatwg/html#7361
> whatwg/html#8174
>
> The gist of the changes are:
> 1. Use the DOM tree instead of the flat tree to search for an element to
>    give initial focus.
> 2. Don't traverse into shadow roots when looking for elements to give
>    initial focus unless the shadow root has delegatesFocus.
>
> This will stay experimental until I have also made the other changes for
> dialog initial focus: whatwg/html#8199
> After those changes have been made as well, I will carefully enable the
> flag by default.
>
> Fixed: 383230, 670130, 1292852
> Change-Id: I13995197f1942aa356cab0f3b41a0e226d1d170d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3961528
> Reviewed-by: Mason Freed <masonf@chromium.org>
> Commit-Queue: Joey Arhar <jarhar@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1067796}

Change-Id: I010be9a77fd8c289edb8c028355182d0fe7dec39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4005547
Commit-Queue: Tim Sergeant <tsergeant@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Tim Sergeant <tsergeant@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Tim Sergeant <tsergeant@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1067950}

html/semantics/interactive-elements/the-dialog-element/dialog-focus-shadow.html

@@ -214,24 +214,6 @@
   </template>
 </dialog>
 
-<dialog data-description="No autofocus, no delegatesFocus, slotted target">
-  <template class="turn-into-shadow-tree">
-    <button tabindex="-1">Focusable</button>
-    <slot></slot>
-    <button tabindex="-1">Focusable</button>
-  </template>
-  <button tabindex="-1" class="focus-me">Focusable</button>
-</dialog>
-
-<dialog data-description="Shadowroot on child, no autofocus, no delegatesFocus">
-  <div>
-    <template class="turn-into-shadow-tree">
-      <button tabindex="-1">Focusable</button>
-    </template>
-  </div>
-  <button tabindex="-1" class="focus-me">Focusable</button>
-</dialog>
-
 <script>
 for (const template of document.querySelectorAll(".turn-into-shadow-tree")) {
   const div = document.createElement("div");