[Fuchsia] Really remove dead GN flag

BUILD.gn

 # Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # This target will be built if no target is specified when invoking ninja.
 group("default") {
   if (is_fuchsia || is_fuchsia_host) {
     # Fuchsia has run_vm_tests marked testonly.
     testonly = true
   }
@@ skipping 103 matching lines @@
 
 group("samples") {
   deps = [
     "runtime/bin:sample_extension",
   ]
 }
 
 # The rules below build a Fuchsia OS image that includes the Dart tree
 # under /system/test/dart.
 if (is_fuchsia) {
-  declare_args() {
-    dart_build_fuchsia_test_image = false
-  }
-
   action("generate_dart_test_manifest") {
     testonly = true
 
     deps = [
       "//packages/gn:mkbootfs",
     ]
 
     output_prefix = "$target_gen_dir/dart_test_tree"
     outputs = [
       "$output_prefix.manifest",
@@ skipping 60 matching lines @@
       rebase_path(input),
       "--output-file",
       rebase_path(output),
       "--build-id-map",
       rebase_path("$target_gen_dir/build_id_map"),
       "--pre-binaries",
       rebase_path(magenta_bootdata),
     ]
   }
 }