[iOS] Remove NewClearBrowsingDataUI experiment

Remove NewClearBrowsingDataUI experiment, as it has become the default.

Bug: 947456
Change-Id: I41f1b2eb5824d18f77bea3447b7b79bfb1469eff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2039410Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Huanzhong Huang <huanzhong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739231}

testing/variations/fieldtrial_testing_config.json

@@ -4001,21 +4001,6 @@
             ]
         }
     ],
-    "NewClearBrowsingDataUI": [
-        {
-            "platforms": [
-                "ios"
-            ],
-            "experiments": [
-                {
-                    "name": "NewClearBrowsingDataUI",
-                    "enable_features": [
-                        "NewClearBrowsingDataUI"
-                    ]
-                }
-            ]
-        }
-    ],
     "NewTabInProductHelp": [
         {
             "platforms": [