low-prio: implement delete subcommand

[peb-adr]

This is - at least currently - more an aesthetic issue and came up when thinking about #181 .
When we have the generic commands create, get, set , we have CRU out of the the CRUD operations implemented.
So for the sake of completeness, eventually implementing a delete command as well would be desirable.
But at the moment no actual need for this is present.

It probably should look something like this:
`openslides delete "{"id": 42}"