[_fe_analyzer_shared] Remove no-longer-needed docImport.

This paves the way for a follow-up CL that will remove the imported
file.

Change-Id: I6a6a69640d09e354a5a5ed1f3f0cce09722fce31
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/461801
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/messages/codes.dart

@@ -2,7 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-/// @docImport 'package:analyzer/src/dart/error/syntactic_errors.dart';
 /// @docImport 'package:analyzer/src/fasta/ast_builder.dart';
 /// @docImport 'package:analyzer/src/fasta/error_converter.dart';
 /// @docImport 'package:analyzer/src/diagnostic/diagnostic_code_values.dart';