cleanup: Remove use_openh264 build flag

This flag doesn't appear to be affecting anything, it certainly
doesn't stop OpenH264 from being used in media or RTC.

All it does is control whether OpenH264 is included into artificial
testonly target "gn_all". Its value is false by default, and nobody
seems to set it to true.

Bug: 1158845
Change-Id: I27bdf215743d81aa65b643e1d3340345e7e56910
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2599111Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Sergey Silkin <ssilkin@google.com>
Commit-Queue: Eugene Zemtsov <eugene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838629}

BUILD.gn

@@ -26,7 +26,6 @@ import("//media/gpu/args.gni")
 import("//media/media_options.gni")
 import("//remoting/remoting_enable.gni")
 import("//third_party/closure_compiler/compile_js.gni")
-import("//third_party/openh264/openh264_args.gni")
 import("//tools/ipc_fuzzer/ipc_fuzzer.gni")
 import("//ui/base/ui_features.gni")
 import("//ui/gl/features.gni")
@@ -684,14 +683,6 @@ group("gn_all") {
     ]
   }
 
-  if (use_openh264) {
-    deps += [
-      "//third_party/openh264:common",
-      "//third_party/openh264:encoder",
-      "//third_party/openh264:processing",
-    ]
-  }
-
   if (enable_message_center) {
     deps += [ "//ui/message_center:message_center_unittests" ]
   }

third_party/openh264/BUILD.gn

@@ -5,7 +5,6 @@
 import("//build/config/chromeos/ui_mode.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//third_party/nasm/nasm_assemble.gni")
-import("//third_party/openh264/openh264_args.gni")
 import("//third_party/openh264/openh264_sources.gni")
 
 # Config shared by all openh264 targets.

third_party/openh264/openh264_args.gni

-# 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("//build/config/chrome_build.gni")
-
-declare_args() {
-  # Enable this to build OpenH264 (for encoding, not decoding).
-  # CHECK THE OPENH264 LICENSE/PATENT BEFORE BUILDING, see
-  # http://www.openh264.org/.
-  use_openh264 = false
-}