Remove null safety codelab for now

firebase.json

@@ -82,6 +82,7 @@
       { "source": "/cl", "destination": "https://dart-review.googlesource.com/q/status:open+-is:wip", "type": 301 },
       { "source": "/cl/:rest*", "destination": "https://dart-review.googlesource.com/c/sdk/+/:rest*", "type": 301 },
       { "source": "/cloud{,/**}", "destination": "/server/google-cloud", "type": 301 },
+      { "source": "/codelabs/null-safety{,/**}", "destination": "/null-safety/understanding-null-safety", "type": 301 },
       { "source": "/codelabs/server{,/**}", "destination": "/tutorials/server/httpserver", "type": 301 },
       { "source": "/code-of-conduct", "destination": "/community/code-of-conduct", "type": 301 },
       { "source": "/concurrency", "destination": "/language/concurrency", "type": 301 },
@@ -225,7 +226,7 @@
       { "source": "/mailing-list", "destination": "https://groups.google.com/a/dartlang.org/forum/#!forum/misc", "type": 301 },
       { "source": "/mobile", "destination": "/multiplatform-apps", "type": 301 },
       { "source": "/news{,/**}", "destination": "https://medium.com/dartlang", "type": 301 },
-      { "source": "/null-safety/tour", "destination": "/codelabs/null-safety", "type": 301 },
+      { "source": "/null-safety/tour", "destination": "/null-safety/understanding-null-safety", "type": 301 },
       { "source": "/observatory{,/**}", "destination": "/tools/dart-devtools", "type": 301 },
       { "source": "/packages", "destination": "https://pub.dev", "type": 301 },
       { "source": "/platforms", "destination": "/overview#platform", "type": 301 },

src/_data/side-nav.yml

@@ -16,8 +16,6 @@
           permalink: /codelabs/iterables
         - title: Asynchronous programming
           permalink: /codelabs/async-await
-        - title: Null safety
-          permalink: /codelabs/null-safety
 - title: Language
   expanded: false
   children:

src/content/codelabs/index.md

@@ -31,10 +31,6 @@ Discover Dart 3's new records and patterns features.
 Learn how you can use them in a Flutter app to help you
 write more readable and maintainable Dart code.
 
-### [Null safety](/codelabs/null-safety)
-
-Use DartPad to learn about Dart's null-safe type system.
-
 ## Flutter
 
 To learn about Flutter, try one of the