Revert "Re-enable testOpenManyTabsQuickly"

This reverts commit d42c900a.

Reason for revert: Flaking on android-pie-x86-rel:
https://ci.chromium.org/p/chromium/builders/ci/android-pie-x86-rel/2682

Original change's description:
> Re-enable testOpenManyTabsQuickly
>
> Bug: 223587
> Change-Id: I1b61f97f9c864d467593c490fa4fc5b7efd16989
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493001
> Reviewed-by: Theresa  <twellington@chromium.org>
> Commit-Queue: Matthew Jones <mdjones@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#820745}

TBR=twellington@chromium.org,mdjones@chromium.org

Change-Id: Ifabbdbba158a3e9b6d49b696c5c1391cdfbda701
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 223587
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498949Reviewed-by: Patrick Noland <pnoland@chromium.org>
Commit-Queue: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820803}

chrome/android/javatests/src/org/chromium/chrome/browser/TabsTest.java

@@ -606,7 +606,7 @@ public class TabsTest {
      *
      */
     @Test
-    @LargeTest
+    @FlakyTest
     public void testOpenManyTabsQuickly() {
         int startCount = mActivityTestRule.getActivity().getCurrentTabModel().getCount();
         for (int i = 1; i <= STRESSFUL_TAB_COUNT; ++i) {