Revert "DevTools: Add ARIAUtils.setCheckboxAsIndeterminate"

This reverts commit 2cf002af.

Reason for revert: Breaks http/tests/devtools/a11y-axe-core/basic-a11y-test.js and 5 other(s), see issue 1004150 - locally identified
tests as fixed when reverting this CL (on Linux)

Original change's description:
> DevTools: Add ARIAUtils.setCheckboxAsIndeterminate
>
> This adds a new API to UI.ARIAUtils to set an element's aria-checked
> attribute to 'mixed', which is the equivalent of setting
> intedeterminate=true on an actual checkbox element.
> This is needed to support keyboard accessibility in the javascript and
> event listener breakpoints views.
>
> Equivalent to the Animation checkbox in this screenshot:
> https://gyazo.com/99f2067f6810af3366943176393d8b2f
>
> Bug: 963183
> Change-Id: Ie09f8e993d1776774c410f6099356f3a1d912048
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715815
> Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
> Auto-Submit: Jack Lynch <jalyn@microsoft.com>
> Commit-Queue: Jack Lynch <jalyn@microsoft.com>
> Cr-Commit-Position: refs/heads/master@{#696551}

TBR=pfeldman@chromium.org,einbinder@chromium.org,jalyn@microsoft.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 963183, 1004150
Tbr: ortuno@
Change-Id: I3ef2a4a5b74be00de669db215f27c2586acba40f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803937
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696700}

third_party/blink/renderer/devtools/front_end/ui/ARIAUtils.js

@@ -236,13 +236,6 @@ UI.ARIAUtils.setChecked = function(element, value) {
   element.setAttribute('aria-checked', !!value);
 };
 
-/**
- * @param {!Element} element
- */
-UI.ARIAUtils.setCheckboxAsIndeterminate = function(element) {
-  element.setAttribute('aria-checked', 'mixed');
-};
-
 /**
  * @param {!Element} element
  * @param {boolean} value