Remove deprecated migration scripts (GoogleCloudPlatform#2119)

* Remove deprecated migration scripts

* Comment out unused imports to pass linter

* Remove commented out imports

* Remove deprecated migration scripts

* Add readme pointing to env upgrade docs

Author: leahecole

composer/tools/README.md

@@ -0,0 +1,3 @@
+# Composer Tools
+
+At this time, there are no Composer tools on GitHub. The environment migration script previously located here has been deprecated with the release of [in-place version upgrades](https://cloud.google.com/composer/docs/how-to/managing/upgrading).