ui: remove deprecated 'password' option for SPICE

This has been replaced by the 'password-secret' option,
which references a 'secret' object instance.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>