Cleanup support for old versions of package:dwds

[nshahan]

In the next major version of the Dart SDK (3.0.0) it should be safe to remove the backwards compatibility for versions of package:dwds older than v11.1.0.

This means we can completely remove the support for writing 'dart.developer.registerExtension' and 'dart.developer.postEvent' to the chrome debug console.

Newer versions of package:dwds already communicate these events through function hooks. See c83eeac.

[nshahan]

cc @elliette
Tracking the cleanup we briefly discussed on the DDC side. It turns out I actually already did the cleanup on the dwds side. 😆 dart-lang/webdev#1342