Remove null safety codelab for now (dart-lang#5539)

Removes the null safety codelab and references to it. In a few days, I
will follow up soon with some new samples on the cheatsheet to replace
this.

This codelab is the least viewed on the site and no longer makes sense
to separate from other language learning. As mentioned, I will follow up
with a few new samples on the cheatsheet as a stop-gap until we
implement a consolidated new-learner experience.

Contributes to dart-lang#5382
Contributes to dart-lang#4603
Closes dart-lang#3093

Author: parlough

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