Revert "Clear the cache in AutomationManagerAura when disabled."

This reverts commit 2d1e6ddf.

Reason for revert: Broke tast tests by crashing chrome

CQ_INCLUDE_TRYBOTS=luci.chrome.try:chromeos-betty-pi-arc-chrome
CQ_INCLUDE_TRYBOTS=luci.chrome.try:chromeos-eve-chrome
CQ_INCLUDE_TRYBOTS=luci.chrome.try:chromeos-kevin-chrome

Original change's description:
> Clear the cache in AutomationManagerAura when disabled.
>
> On the Tast tests which use the Automation features, I'm seeing
> sometimes the automation node tree gets empty. During the debugging,
> I've noticed that sometimes the automation tree fails to build
> the tree updates (happens OnSerializeFailure).
>
> This can be resolved by clearing the cache when it is disabled
> (which happens regularly on tests).
>
> Bug: 1165292
> Test: on kevin64, run informational Tast tests
> AX-Relnotes: n/a
> Change-Id: I835e06ca2118ae6c36d6fb8b877fad074bd41fcf
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627715
> Commit-Queue: Jun Mukai <mukai@chromium.org>
> Reviewed-by: David Tseng <dtseng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#845868}

TBR=mukai@chromium.org,dtseng@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: I43a2458db101f777ea81537127db2de2a85ca757
AX-Relnotes: n/a
Bug: 1165292, 1169573
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2642843
Commit-Queue: Fergus Dall <sidereal@google.com>
Reviewed-by: Fergus Dall <sidereal@google.com>
Cr-Commit-Position: refs/heads/master@{#846094}

chrome/browser/ui/aura/accessibility/automation_manager_aura.cc

@@ -75,8 +75,6 @@ void AutomationManagerAura::Enable() {
 
 void AutomationManagerAura::Disable() {
   enabled_ = false;
-  cache_ = std::make_unique<views::AXAuraObjCache>();
-  tree_.reset();
   Reset(true);
 }