sso-proxy: remove deprecated session unmarshal code and old cookie secrets

[shrayolacrayon]

Why
We have two code paths for loading the session state in sso-proxy that we kept in until the cookie refresh period expired for sso in production. Both code paths were kept in place so that having an invalidly decrypted session cookie would not affect the UX for those logging into sso-proxy with the sessions encrypted the old way. We can now remove that code path.

This will involve removing:

• OldCookieCipher and OldCookieSecret from the OAuthProxy struct.
• [initialization logic] for old cookie secrets and ciphers (https://github.com/buzzfeed/sso/blob/master/internal/proxy/oauthproxy.go#L249) in NewOAuthProxy
• ErrRefreshCookie
• fallback of using old cookie cipher in LoadCookiedSession

[loganmeetsworld]

Handled in #54