configure: remove help string for 'vnc-tls' option

The '--enable-vnc-tls' option to configure was removed in

  commit 3e305e4
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Thu Aug 6 14:39:32 2015 +0100

    ui: convert VNC server to use QCryptoTLSSession

This removes the corresponding help string.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

configure

@@ -1348,7 +1348,6 @@ disabled with --disable-FEATURE, default is enabled if available:
   vte             vte support for the gtk UI
   curses          curses UI
   vnc             VNC UI support
-  vnc-tls         TLS encryption for VNC server
   vnc-sasl        SASL encryption for VNC server
   vnc-jpeg        JPEG lossy compression for VNC server
   vnc-png         PNG compression for VNC server