[analyzer] Remove ErrorClassInfo.superclass.

This field's default value, `DiagnosticCode`, was never
overridden. It's cleaner to just inline that value in the one place it
was used.

Change-Id: I6a6a6964b16f72486ad21c3f343ce91504c44be6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445524
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>

Author: stereotype441

pkg/analyzer/tool/messages/error_code_info.dart

@@ -532,9 +532,6 @@ class ErrorClassInfo {
   /// based on the [type] of the error.
   final String? severity;
 
-  /// The superclass of this class.
-  final String superclass;
-
   /// The type of errors in this class.
   final String type;
 
@@ -558,7 +555,6 @@ class ErrorClassInfo {
     this.includeCfeMessages = false,
     required this.name,
     this.severity,
-    this.superclass = 'DiagnosticCode',
     required this.type,
     this.deprecatedSnakeCaseNames = const {},
     this.includeInDiagnosticCodeValues = true,

pkg/analyzer/tool/messages/generate.dart

@@ -142,7 +142,7 @@ library;
           out.writeln('/// $line');
         });
       }
-      out.write('class ${errorClass.name} extends ${errorClass.superclass} {');
+      out.write('class ${errorClass.name} extends DiagnosticCode {');
       var entries = [
         ...analyzerMessages[errorClass.name]!.entries,
         if (errorClass.includeCfeMessages)