Do not separate dhstore backends and non-dhstore backends.

[gammazero]

Now that dhstore supports non-encrypted lookups (dhfind), there is no need to separate dhstore backends and not-dhstore backends, unless we intend to allow mixed dh and non-dh storage deployments.

[gammazero]

We need to keep the ability to specify separate backends as long as we continue to provide support for both double-hashed and non-double-hashed storage. There are at least two situations where an indexer operator may need this:

1. When maintaining existing indexers with non-double-hashed storage and adding new indexers that do double hashed indexing.
2. When assigning some publishers to indexers that use double-hashed storage, and other publisher to indexers that do not.