Remove reference to PasswordBranding

PasswordBranding was an experiment to change how Chrome's password
manager is called. It has been removed from Chrome over a year ago [1],
but the last reference in the test config was forgotten. This CL removes
it.

[1] In 57.0.2958.0, by https://codereview.chromium.org/2579823003.

Bug: 799180
Change-Id: Ic003dcb375c490e5d72be6d495962e9860e8bbb1
Reviewed-on: https://chromium-review.googlesource.com/1061694Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559158}

testing/variations/fieldtrial_testing_config.json

@@ -2453,23 +2453,6 @@
             ]
         }
     ],
-    "PasswordBranding": [
-        {
-            "platforms": [
-                "android",
-                "chromeos",
-                "ios",
-                "linux",
-                "mac",
-                "win"
-            ],
-            "experiments": [
-                {
-                    "name": "SmartLockBrandingSavePromptOnly"
-                }
-            ]
-        }
-    ],
     "PasswordGeneration": [
         {
             "platforms": [