chore(rln-relay): remove registration capabilities

[rymnc]

Background

Currently, rln-relay has 2 functions -

1. Perform rate limiting on rln-relay protected topics
2. Allow a user to register into the membership set

Details

As discussed in the offsite with @alrevuelta, it does not make sense to retain the "registration" capabilities in the rln-relay module, rather, we would expect users to obtain their membership out of band, via the application-specific registration criteria.

We should just accept keystore OR cli args with their membership details, and not provide registrations to our users.

Acceptance criteria

• Remove registration from the OnChainGroupManager
• Remove the private key cli arg to wakunode2/chat2

Need thumbs from @jm-clius here!

[jm-clius]

👍 for general direction, but wouldn't this be good to keep as a (perhaps hidden) dev feature?

[vpavlin]

I'd agree, maybe remove from CLI, keep in code?

[rymnc]

Not sure if we want to keep it altogether, if we're trying to cut scope as much as possible.
The current requirement for waku-rln-relay is that we permission a set of users into the membership set.
cc: @alrevuelta