(dev/core#2258) CryptoRegistry - Remove fallback option based on CIVICRM_SITE_KEY.

(dev/core#2258) CryptoRegistry - Remove fallback option based on CIVICRM_SITE_KEY.

This was included under the expectation it might make a nicer upgrade. But
I don't think it buys a whole lot:

1. You run the upgrader. The SMTP password is converted from
   rj256-ecb-sitekey to aes-cbc-sitekey.  All other credentials are left
   unencrypted.  Afterward, you set CIVICRM_CRED_KEY and run the rekey.

2. You run upgrader. The SMTP password is decrypted. All other credentials
   are left unencrypted. Afterward, you set CIVICRM_CRED_KEY and run the rekey.

Additionally, I think there's a question of risk-management when we get to
encrypting more things in the Setting and API layers.  If we go with path 2,
then we can ramp-up adoption progressively, e.g.

* Release 1: Add support as non-default option
* Release 2: Enable by default on new builds
* Release 3: Display alert to existing sites that don't have encryption keys