Fix compile_from_dill test.

I recently changed the FE in how it loads sdk files. That internally fixed the
meaning of this flag and made it unnecessary for our script.

TBR=efortuna@google.com

Review-Url: https://codereview.chromium.org/2999553002 .

pkg/compiler/tool/generate_kernel.dart

@@ -22,7 +22,6 @@ main(List<String> args) async {
   var options = new CompilerOptions()
     ..target = new Dart2jsTarget(new TargetFlags())
     ..packagesFileUri = Uri.base.resolve('.packages')
-    ..compileSdk = true
     ..setExitCodeOnProblem = true
     ..linkedDependencies = [Uri.base.resolve(flags['platform'])];