Removing 'automatically generated' disclaimer from Abseil BUILD.gn files.

The first version of these files was automatically generated, now I
manually keep them up to date each time I roll Abseil because there are
nontrivial mismatches between Bazel and Chromium environments.

Bug: None
Change-Id: I4984d43cbbe3bc87de3bd862628179ba301aa32f
Reviewed-on: https://chromium-review.googlesource.com/1102568
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567742}

third_party/abseil-cpp/absl/algorithm/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/base/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/container/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/debugging/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 

third_party/abseil-cpp/absl/memory/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/meta/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/numeric/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/strings/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/synchronization/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/time/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/time/internal/cctz/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/types/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {

third_party/abseil-cpp/absl/utility/BUILD.gn

@@ -2,8 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# WARNING: This file is automatically generated.
-
 import("//build_overrides/build.gni")
 
 if (build_with_chromium) {