Remove code related to the PreReadChromeChildInBrowser field trial.

testing/variations/fieldtrial_testing_config_win.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 293 matching lines @@
         }
     ],
     "PointerEvent": [
         {
             "enable_features": [
                 "PointerEvent"
             ],
             "group_name": "Enabled"
         }
     ],
-    "PreRead": [
-        {
-            "group_name": "NoPrefetchArgument2",
-            "params": {
-                "NoPrefetchArgument": "true"
-            }
-        },
-        {
-            "group_name": "PreReadChromeChildInBrowser",
-            "params": {
-                "PreReadChromeChildInBrowser": "true"
-            }
-        }
-    ],
     "PreconnectMore": [
         {
             "enable_features": [
                 "PreconnectMore"
             ],
             "group_name": "Enabled"
         }
     ],
     "QUIC": [
         {
@@ skipping 205 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }