Remove ChromeVox listener for text changes

Previously, we used this event to make up for gaps in support for text area
events (not receiving events for some changes in the caret).

However, ChromeVox now provides feedback for text areas using document selection
changes.

Furthermore, since
https://chromium-review.googlesource.com/c/chromium/src/+/2379091

the task manager UI is once again usable. However, because the UI fires a lot of
text changes, the experience is quite slow. Without listening to these events,
the UI is again usable.

R=akihiroota@chromium.org, nektar@chromium.org

Bug: none
Change-Id: I7e7aa7aae761b271f36754b5074971a23b5d1cac
AX-Relnotes: Makes the Task Manager (shift+escape) usable with ChromeVox.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462338Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815399}

chrome/browser/resources/chromeos/accessibility/chromevox/background/desktop_automation_handler.js

@@ -85,7 +85,6 @@ DesktopAutomationHandler = class extends BaseAutomationHandler {
     // Called when a same-page link is followed or the url fragment changes.
     this.addListener_(EventType.SCROLLED_TO_ANCHOR, this.onScrolledToAnchor);
     this.addListener_(EventType.SELECTION, this.onSelection);
-    this.addListener_(EventType.TEXT_CHANGED, this.onEditableChanged_);
     this.addListener_(
         EventType.TEXT_SELECTION_CHANGED, this.onEditableChanged_);
     this.addListener_(EventType.VALUE_CHANGED, this.onValueChanged);
@@ -370,7 +369,6 @@ DesktopAutomationHandler = class extends BaseAutomationHandler {
     const isContentEditable = evt.target.state[StateType.RICHLY_EDITABLE];
 
     switch (evt.type) {
-      case EventType.TEXT_CHANGED:
       case EventType.TEXT_SELECTION_CHANGED:
       case EventType.VALUE_CHANGED:
         // Known to be duplicated by document selection changes for content