postfix: deprecated `sslCACert` in favour of `tlsTrustedAuthorities`

`sslCACert` was used for trust store of client and server certificates. Since `smtpd_tls_ask_ccert` defaults to no the setup of `smtpd_tls_CApath` was removed.

>By default (see smtpd_tls_ask_ccert), client certificates are not requested, and smtpd_tls_CApath should remain empty.
see http://www.postfix.org/postconf.5.html#smtpd_tls_CAfile