Remove the google/dart-appengine Docker image

We will not support this for 1.8, see https://groups.google.com/a/dartlang.org/forum/?utm_medium=email&utm_source=footer#!topic/cloud-trusted-testers/cdsYhewn9to.

R=kustermann@google.com, wibling@google.com
BUG=

Review URL: https://codereview.chromium.org//758933003