No longer build "generate_dart_ui" build target in flutter recipe

The flutter-hhh build is failing due to removal of this target.
The target was removed here:

    https://github.com/flutter/engine/pull/27445

Change-Id: Id06abdda12641878d7707f8a229d38efb35419dd
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/207181
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
6 files changed