Delete `build/module_args` directory (#898)

Part 2 of https://github.com/flutter/engine/pull/55441.

build/module_args/dart.gni

diff --git a/build/module_args/dart.gni b/build/module_args/dart.gni
deleted file mode 100644
index ee6b038..0000000
--- a/build/module_args/dart.gni
+++ /dev/null
@@ -1,6 +0,0 @@
-# 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.
-
-# This variable should point to the Dart SDK.
-dart_sdk_root = "//third_party/dart-sdk/dart-sdk"

build/module_args/nacl.gni

diff --git a/build/module_args/nacl.gni b/build/module_args/nacl.gni
deleted file mode 100644
index 61e0768..0000000
--- a/build/module_args/nacl.gni
+++ /dev/null
@@ -1,6 +0,0 @@
-# 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.
-
-# Override nacl's build directory.
-nacl_shared_build_dir = "//build"