Remove field trial config for PrioritizeCompositingUntilBeginMainFrame.

This feature has been enabled by default.

Bug: 966177
Change-Id: Ib41a4cfb721d9cc5115de8c5569112493b1cc4e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490921
Commit-Queue: Katie Dillon <kdillon@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#822806}

testing/variations/fieldtrial_testing_config.json

@@ -1190,25 +1190,6 @@
             ]
         }
     ],
-    "BlinkSchedulerPrioritizeRenderingUntilBeginMainFrame": [
-        {
-            "platforms": [
-                "android",
-                "chromeos",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "BlinkSchedulerPrioritizeCompositingUntilBeginMainFrame"
-                    ]
-                }
-            ]
-        }
-    ],
     "BrowserJankinessProfiling": [
         {
             "platforms": [