Migrate to memcached@.service

[jernst]

... once uboslinux/ubos-packages#30 is closed.

[ethn]

That's closed. Do I need to do anything on this?

[ethn]

@jernst are there still todos here?

[jernst]

Yes, but not critical only "cleanup". Instead of using its own .service file, decko should now use the default memcached@.service file that hadn't existed at the time.