fix(coreos-base/update_engine): remove the au-key

Update engine, rightfully, expects that if there is an au-key then it
should use it. This is a sound theory and we insert the key when we sign
anyways so remove this key for now.
Merge pull request #85 from philips/remove-key-from-default-build

fix(coreos-base/update_engine): remove the au-key