Use the platform -std= for sdk_version.

Bug: http://b/72571399
Test: builds
Change-Id: I294cfadb7de54b1ae648e02ac9af34ed7a7405d8
diff --git a/cc/stl.go b/cc/stl.go
index c5757cd..2da6471 100644
--- a/cc/stl.go
+++ b/cc/stl.go
@@ -176,8 +176,7 @@
 		ndkSrcRoot := android.PathForSource(ctx, "prebuilts/ndk/current/sources/cxx-stl/system/include")
 		flags.CFlags = append(flags.CFlags, "-isystem "+ndkSrcRoot.String())
 	case "ndk_libc++_shared", "ndk_libc++_static":
-		// TODO(danalbert): This really shouldn't be here...
-		flags.CppFlags = append(flags.CppFlags, "-std=c++11")
+		// Nothing.
 	case "":
 		// None or error.
 		if !ctx.toolchain().Bionic() {