Revert "[iOS] Enable the new Clear Browsing Data UI flag"

This reverts commit 684ec4b3.

Reason for revert: Breaks internal egtests

Original change's description:
> [iOS] Enable the new Clear Browsing Data UI flag
> 
> This change is to enable the new-clear-browsing-data-ui flag.
> 
> The new Clear Browsing Data dialog adds the time range selection functionality
> and shows browsing data volume details for each data type.
> Launch Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=627147
> Screenshots: https://drive.google.com/open?id=1q1zJzB1abRsY1zCD4rLewvMxCnMr8WGf
> 
> Bug: 627147
> Change-Id: I6d44d888697e5d03c71c4eacb07dcca6c84e2f88
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1556802
> Reviewed-by: Rohit Rao <rohitrao@chromium.org>
> Commit-Queue: Huanzhong Huang <huanzhong@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#661436}

TBR=rohitrao@chromium.org,huanzhong@chromium.org

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

Bug: 627147
Change-Id: I65f0dc5eba27f78971364ae39c396382b2fc8d46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623493Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661991}

ios/chrome/browser/browsing_data/browsing_data_features.cc

@@ -5,7 +5,7 @@
 #include "ios/chrome/browser/browsing_data/browsing_data_features.h"
 
 const base::Feature kNewClearBrowsingDataUI{"NewClearBrowsingDataUI",
-                                            base::FEATURE_ENABLED_BY_DEFAULT};
+                                            base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kWebClearBrowsingData{"WebClearBrowsingData",
                                           base::FEATURE_DISABLED_BY_DEFAULT};