[Fuchsia] Really remove dead GN flag

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index d090a986939f91b34deb52e6853161a36f25a095..6b0eb2ed56f3be3f500ced9698e8b7c44c7639e2 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -121,10 +121,6 @@ group("samples") {
 # 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