Revert "Add field trial config for BackgroundResourceFetch"

This reverts commit aeb8508.

Reason for revert: Caused unexpected regression (crbug.com/1502040).

Bug: 1502040

Original change's description:
> Add field trial config for BackgroundResourceFetch
>
> Bug: 1379780
> Change-Id: I6c76bd16bd4a02a17a1b3d2a0bcfe6a934bf5134
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5018027
> Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
> Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1224008}

Bug: 1379780
Change-Id: If50528c3d9f915e415b17cfc5b9a3fa6540d8321
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5030352
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1224666}

testing/variations/fieldtrial_testing_config.json

@@ -2293,27 +2293,6 @@
             ]
         }
     ],
-    "BackgroundResourceFetch": [
-        {
-            "platforms": [
-                "android",
-                "android_webview",
-                "chromeos",
-                "chromeos_lacros",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "BackgroundResourceFetch"
-                    ]
-                }
-            ]
-        }
-    ],
     "BackgroundTabLoadingFromPerformanceManager": [
         {
             "platforms": [