Revert "[iOS] Have the BVC being contained by default"

This reverts commit f561a5d4.

Reason for revert: speculative revert to fix ios_chrome_ui_eg2tests_module failure on ios-simulator-noncq

Sample failure log: https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8887351179385452176/+/steps/ios_chrome_integration_eg2tests_module__iPhone_7_iOS_13.3___with_patch__on_Mac-10.14.6/0/logs/UKMTestCase__x2f_testSingleSyncSignout/0
---
Exception with Action: {
  "Action Name" : "Swipe Down for duration 1",
  "Element Matcher" : "web view scroll view",
  "Recovery Suggestion" : "Check if the element exists in the UI hierarchy printed below. If it exists, adjust the matcher so that it accurately matches element.",
  "Search API Info" : ""
}
---

Original change's description:
> [iOS] Have the BVC being contained by default
> 
> This enables the flag by default on Canary.
> The idea is to ship it to 100%, with a kill switch by Finch.
> 
> Bug: 1038034
> Change-Id: I0ea6097bd844db1940eff683d99d555f1654189d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062979
> Commit-Queue: Gauthier Ambard <gambard@chromium.org>
> Reviewed-by: Mark Cogan <marq@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#744811}

TBR=marq@chromium.org,gambard@chromium.org

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

Bug: 1038034
Change-Id: If13fa3134ac0bddd74b0e12579ab059dfbbb6084
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078814Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/master@{#745345}

ios/chrome/browser/ui/ui_feature_flags.cc

@@ -29,7 +29,7 @@ const base::Feature kForceUnstackedTabstrip{"ForceUnstackedTabstrip",
                                             base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kContainedBVC{"ContainedBVC",
-                                  base::FEATURE_ENABLED_BY_DEFAULT};
+                                  base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kTestFeature{"TestFeature",
                                  base::FEATURE_DISABLED_BY_DEFAULT};