remove stale TODO

As per discussion (https://github.com/dart-lang/linter/issues/4055#issuecomment-1568734934), considering existing docs sufficient for now.

Change-Id: Idcdaa5f4c238199e8e755237662f92023986545d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/379000
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

pkg/linter/lib/src/rules/invalid_case_patterns.dart

@@ -236,8 +236,6 @@ switch (n) {
 ```
 ''';
 
-// TODO(pq): add a dart.dev doc link when a doc is final (https://github.com/dart-lang/linter/issues/4055)
-
 class InvalidCasePatterns extends LintRule {
   static const LintCode code = LintCode('invalid_case_patterns',
       "This expression is not valid in a 'case' clause in Dart 3.0.",