Remove base64 encoding support on secrets GET responses

We are having issues supporting the base64 encoding feature when getting decrypted secrets. In particular the Accept-Encoding http header appears to be overridden by browsers such as Chrome and the 'requests' library used by Cloud Cafe, even though 'base64' is set for header. Rather than contend with this ill-supported header in the long term, it is better to just remove this feature for the time being.