Now that configuration records use the 'sanitize' property for each
field, 'validate-configuration' has become useless because it's
impossible to construct an invalid configuration record.
* gnu/services/configuration.scm (validate-configuration): Remove.
* gnu/services/mail.scm (dovecot-service): Remove call.
* gnu/services/vpn.scm (openvpn-client-service)
(openvpn-server-service): Likewise.
* doc/guix.texi (Complex Configurations): Remove documentation.