linter: remove unused State.getDescription

Change-Id: Ia241ed01ad81cfdc399cf3011e3b4f6d43113950
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/428926
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/lint/state.dart

@@ -92,11 +92,6 @@ sealed class State {
   /// A short description, suitable for displaying in documentation or a
   /// diagnostic message.
   String get label;
-
-  /// An optional description that can be used in documentation or diagnostic
-  /// reporting.
-  @Deprecated('Not set by any lint rule, remove any usages.')
-  String? getDescription() => null;
 }
 
 extension StateExtension on State {