configure: Drop vnc-ws feature from help text

Commit 8e9b0d24 (ui: convert VNC websockets to use crypto APIs) dropped
the --enable-vnc-ws option but forgot to update the help text. Fix this.

Cc: Daniel P. Berrange <berrange@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1437749257-3313-1-git-send-email-afaerber@suse.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>