Remove all code related to the PreRead field trial.

components/startup_metric_utils/common/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
-source_set("common") {
-  sources = [
-    "pre_read_field_trial_utils_win.cc",
-    "pre_read_field_trial_utils_win.h",
-  ]
-
-  deps = [
-    "//base",
-    "//components/variations",
-    "//ipc",
-  ]
-}
-
 mojom("interfaces") {
   sources = [
     "startup_metric.mojom",
   ]
   deps = [
     "//mojo/common:common_custom_types",
   ]
 
   use_new_wrapper_types = false
 }