[omnibox] Clean up On Device Head Suggest fieldtrial test entries

after launching feature to non-Incognito Mobile & Incognito all
platforms.

Bug: 925072
Change-Id: I564833f2051b4b5b04980a119b121f2ee2e6da18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537521
Commit-Queue: Ce Chen <cch@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827726}

testing/variations/fieldtrial_testing_config.json

@@ -4781,44 +4781,9 @@
             ]
         }
     ],
-    "OmniboxOnDeviceHeadSuggestIncognito": [
-        {
-            "platforms": [
-                "android",
-                "windows",
-                "mac",
-                "chromeos",
-                "linux"
-            ],
-            "experiments": [
-                {
-                    "name": "OnDeviceHeadSuggest_Incognito_Enabled",
-                    "enable_features": [
-                        "OmniboxOnDeviceHeadProviderIncognito"
-                    ]
-                }
-            ]
-        }
-    ],
-    "OmniboxOnDeviceHeadSuggestIncognitoIOS": [
-        {
-            "platforms": [
-                "ios"
-            ],
-            "experiments": [
-                {
-                    "name": "OnDeviceHeadSuggest_Incognito_Enabled",
-                    "enable_features": [
-                        "OmniboxOnDeviceHeadProviderIncognito"
-                    ]
-                }
-            ]
-        }
-    ],
     "OmniboxOnDeviceHeadSuggestNonIncognito": [
         {
             "platforms": [
-                "android",
                 "windows",
                 "mac",
                 "chromeos",
@@ -4834,21 +4799,6 @@
             ]
         }
     ],
-    "OmniboxOnDeviceHeadSuggestNonIncognitoIOS": [
-        {
-            "platforms": [
-                "ios"
-            ],
-            "experiments": [
-                {
-                    "name": "OnDeviceHeadSuggest_NonIncognito_Enabled",
-                    "enable_features": [
-                        "OmniboxOnDeviceHeadProviderNonIncognito"
-                    ]
-                }
-            ]
-        }
-    ],
     "OmniboxPreserveDefaultMatchAgainstAsyncUpdate": [
         {
             "platforms": [