Remove obsolete ember-cli-deprecation-workflow addon

We're currently not using this, so we don't need to install it anymore. Additionally, the new major version appears to unconditionally include the addon code in production builds too which is a price I'm not willing to pay.

Author: Turbo87

config/deprecation-workflow.js

@@ -86,7 +86,6 @@
     "ember-cli-code-coverage": "3.0.0",
     "ember-cli-dependency-checker": "3.3.2",
     "ember-cli-dependency-lint": "2.0.1",
-    "ember-cli-deprecation-workflow": "2.2.0",
     "ember-cli-head": "2.0.0",
     "ember-cli-htmlbars": "6.3.0",
     "ember-cli-inject-live-reload": "2.1.0",