Remove legacy adapters for 2.0

[kfranqueiro]

dstore 2.0 is intended to be the store implementation for Dojo 2, so it makes sense to drop the legacy adapters.

I had intended to do this earlier and apparently forgot, and already unwittingly cost @maier49 a bit of time in #145 as a result. Sorry about that. We can leave those changes as-is and tackle this once the first round of conversion work is merged (or as part of that branch if work continues there, I suppose).

PS: we probably should remember to prune from the docs as well, while we're at it.