Revert of Allow overriding GN build flags

samples/BUILD.gn

 # Copyright 2014 PDFium 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("//build/config/sanitizers/sanitizers.gni")
 import("//build_overrides/v8.gni")
-import("//build_overrides/pdfium.gni")
+import("../pdfium.gni")
 
 group("samples") {
   testonly = true
   deps = [
     ":pdfium_diff",
     ":pdfium_test",
   ]
 }
 
 config("pdfium_samples_config") {
@@ skipping 60 matching lines @@
   ]
   if (!pdf_enable_xfa) {
     deps += [ "../third_party:fx_lpng" ]
   }
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     ":pdfium_samples_config",
     "//build/config/compiler:no_chromium_code",
   ]
 }