Kill the DynamicDHGroups feature
In #4548 (moved) we implemented the DynamicDHGroups
feature of prop179 which we ended up never using. It's pretty clear now that we will never use it at all, because we have PTs to do this job.
Since the code is non-trivial, I suggest we kill it completely from the current codebase.
Seems like a good volunteer task too.