assistant: remove deprecated launcher exp

Bug: None
Change-Id: I8d6725898a1633a87be4759d76f9a12fe2ea49fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625808
Auto-Submit: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842791}

testing/variations/fieldtrial_testing_config.json

@@ -1658,21 +1658,6 @@
             ]
         }
     ],
-    "ChromeOSLauncherAssistantIntegration": [
-        {
-            "platforms": [
-                "chromeos"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "EnableEmbeddedAssistantUI"
-                    ]
-                }
-            ]
-        }
-    ],
     "ChromeOSNeuralPalm": [
         {
             "platforms": [