Remove unnecessary import from compiler

Bug: #44569
Change-Id: I98e103a1af11794927fc1fe6794e69ec655debf6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/213424
Auto-Submit: Samuel Rawlins <[email protected]>
Commit-Queue: Mayank Patke <[email protected]>
Reviewed-by: Mayank Patke <[email protected]>

Author: srawlins

pkg/compiler/tool/kernel_visitor/dart_html_metrics_visitor.dart

@@ -4,7 +4,6 @@
 import "dart:convert";
 import "dart:io";
 import "package:kernel/kernel.dart";
-import "package:kernel/ast.dart";
 
 main(List<String> args) async {
   // Ensure right args are passed.