Remove UrgentDiscardingFromPerformanceManager entry from testing config.

The Windows study has ended. The Mac study is only on old versions.

Bug: 1126640
Change-Id: I80d2f2c98b976277a8d92f558ef9843cabd63580
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631431Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#844074}

testing/variations/fieldtrial_testing_config.json

@@ -7160,21 +7160,6 @@
             ]
         }
     ],
-    "UrgentDiscardingFromPerformanceManager": [
-        {
-            "platforms": [
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled_20200129",
-                    "enable_features": [
-                        "UrgentDiscardingFromPerformanceManager"
-                    ]
-                }
-            ]
-        }
-    ],
     "UseFluentLanguageModel": [
         {
             "platforms": [