ui/spice: QXLInterface method set_mm_time() is now dead, drop

SPICE_NEEDS_SET_MM_TIME is now always off.  Bury the dead code.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230109190321.1056914-7-armbru@redhat.com>