[messages] Delete switch_to_toplevel_diagnostics.dart.

This was a temporary script to aid in migrating the analyzer (and
related packages) away from using the old diagnostic code constants
(static constants in `DiagnosticCode`-derived classes) and to instead
using the top level diagnostics (in `diagnostic.dart` files).

Now that the script has been run, it is no longer needed.

Change-Id: I6a6a69643df23ae5ecdc3b21c91b60c963597d5b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/461584
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

pkg/analyzer_utilities/pubspec.yaml

@@ -21,7 +21,5 @@ dependencies:
 
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dev_dependencies:
-  analyzer_plugin: any
-  linter: any
   lints: any
   test_reflective_loader: any