securedrop_keyring in defaults vs config.py

The securedrop_keyring was only used to create the directory not in the securedrop/config.py file. Changing the value would of broken the install. In order to keep the config.py the same for the current dev environment just removed it. Think the store, keys and db should be configured the same way but don't think they need to be defined in the defaults. Changing the default value would also have required updating the apparmror profiles.