Remove field trial testing config for SignedExchangeSubresourcePrefetch

It's enabled by default at crrev.com/c/2415709.

Bug: 935267
Change-Id: Icc35a4f8934a209b03b5f405f35cb559a855d845
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2423701Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810047}

testing/variations/fieldtrial_testing_config.json

@@ -6600,26 +6600,6 @@
             ]
         }
     ],
-    "SignedExchangeSubresourcePrefetch": [
-        {
-            "platforms": [
-                "android",
-                "android_webview",
-                "chromeos",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "SignedExchangeSubresourcePrefetch"
-                    ]
-                }
-            ]
-        }
-    ],
     "SimplifiedUrlDisplay": [
         {
             "platforms": [