[docker] Remove Dart SDK dependency from docker.py

The last Dart SDK call from build_push has been removed in
https://github.com/dart-lang/dart_docker/pull/55, so docker.py no longer
needs to provision a Dart SDK.

https://github.com/dart-lang/sdk/issues/40991

Change-Id: I2b796e7e839ac5dedb1fd7f5bb443f0ee3543536
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/142100
Reviewed-by: William Hesse <whesse@google.com>
3 files changed