Remove AddToHomescreenMessaging entry from the testing config.

The server-side config shows that the study has ended.

Bug: 1126640
Change-Id: I76ff1a337b5dad17228ab4c2d2a5b88ca7af37f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632185Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#846140}

testing/variations/fieldtrial_testing_config.json

@@ -33,33 +33,6 @@
             ]
         }
     ],
-    "AddToHomescreenMessaging": [
-        {
-            "platforms": [
-                "android"
-            ],
-            "experiments": [
-                {
-                    "name": "Install app",
-                    "params": {
-                        "include_no_download_required": "false"
-                    },
-                    "enable_features": [
-                        "AddToHomescreenMessaging"
-                    ]
-                },
-                {
-                    "name": "Install app (no download required)",
-                    "params": {
-                        "include_no_download_required": "true"
-                    },
-                    "enable_features": [
-                        "AddToHomescreenMessaging"
-                    ]
-                }
-            ]
-        }
-    ],
     "AmpBackgroundTab": [
         {
             "platforms": [