Remove the client secrets file for Google provider

Terraform seems to work without the client secrets json file. I haven't run all the acceptance tests but I'm able to interact with the API.
Just a small detail here: the packer documentation changes are slightly different (and more correct).  This description should be changed to note that after creating the service account, the user is prompted to download a .p12 file first.  After this, you need to generate a new JSON key.  This key is that should be used for the _account file_.

It took me a few minutes to figure out just what I needed to do here.  It would be nice to make the documentation clear that the first file that is downloaded is not what is needed.  In fact, you can delete the p12 key since that won't be used by terraform.