obsdiscovery: drop `obshash` discovery protocol (issue6136)
The protocol has been superseeded by stablerange for a long time. It is untested
and more and more buggy. Since this is deprecated experimental code in an
experimental code, we drop it.
We keep the underlying computation and debug command around for now. They might
still be useful to looks at repositories
extrakey = 'topic'
changekey = '_rewrite_noise'