rm obsolete comment

Author: Yurii Prykhodko

lint_test/prefer_match_file_name_extension_test.dart

@@ -1,6 +1,5 @@
 // ignore_for_file: unused_element, unused_field
 
 /// Check the `prefer_match_file_name` rule
-/// To trigger lint delete "extension" from exclude_entity in analysis_options.yaml
 // expect_lint: prefer_match_file_name
 extension WrongExtension on List {}