Remove entry for UsePreferredIntervalForVideo from testing config.

The experiment launched. The feature was default enabled on the
appropriate platforms in crrev.com/c/2340110.

Bug: 1126640, 976583
Change-Id: I6142f21d207ab56cd127d3203aea1213f01104a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631930Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#844366}

testing/variations/fieldtrial_testing_config.json

@@ -7158,27 +7158,6 @@
             ]
         }
     ],
-    "UsePreferredIntervalForVideo": [
-        {
-            "platforms": [
-                "chromeos",
-                "mac",
-                "windows",
-                "linux"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "params": {
-                        "NumOfFramesToToggleInterval": "6"
-                    },
-                    "enable_features": [
-                        "UsePreferredIntervalForVideo"
-                    ]
-                }
-            ]
-        }
-    ],
     "UseSkiaRenderer": [
         {
             "platforms": [