chore: remove unused import

Author: incendial

lib/src/analyzers/lint_analyzer/rules/rules_list/prefer_iterable_of/prefer_iterable_of_rule.dart

@@ -2,7 +2,6 @@
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
-import 'package:analyzer/dart/element/nullability_suffix.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:collection/collection.dart';