Reland "[dartdevc] cleaning up unused web files"

This is a reland of e866f04

Original change's description:
> [dartdevc] cleaning up unused web files
>
> Dependent on these google3 changes: https://critique.corp.google.com/#review/272749649
>
> Change-Id: I9e89142cd5b2a619acfc35badb9cf3c549b3be9c
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/119587
> Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
> Commit-Queue: Mark Zhou <markzipan@google.com>

Change-Id: I7e01fb4ad60e47808721fa49b3ca3498e7aeaf46
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/120200
Commit-Queue: Mark Zhou <markzipan@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>

Author: Markzipan

CHANGELOG.md

@@ -82,6 +82,13 @@ main() { foo(() {}); }
 
 ### Dart VM
 
+### Dart for the Web
+
+#### Dart Dev Compiler (DDC)
+
+* Kernel DDC will no longer accept non-dill files as summary inputs.
+* Removed support for the deprecated web extension.
+
 ### Tools
 
 #### Pub