[Passwords] Remove WellKnownChangePassword from Field Trial Config

This change removes WellKnownChangePassword from
fieldtrial_testing_config.json. The feature is fully launched via Finch,
and the feature flag is enabled by default.

Bug: 927473
Change-Id: I8911caedf02b22b1c0f4d466f71487be4268c4af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2571361
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833312}

testing/variations/fieldtrial_testing_config.json

@@ -8147,27 +8147,6 @@
             ]
         }
     ],
-    "WellKnownChangePassword": [
-        {
-            "platforms": [
-                "android",
-                "android_webview",
-                "chromeos",
-                "ios",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "WellKnownChangePassword"
-                    ]
-                }
-            ]
-        }
-    ],
     "WindowsNativeSpellChecker": [
         {
             "platforms": [