nss: Remove cryptographic key support from nss_files, nss_nis, nss_nisplus

nss: Remove cryptographic key support from nss_files, nss_nis, nss_nisplus

The interface has hard-coded buffer sizes and is therefore tied to
DES.  It also does not match current practice where different
services on the same host use different key material.

This change simplifies removal of the sunrpc code.