Consider removing the receivers list from registryDAO

[pasqu4le]

Clarification and motivation

For #137 registryDAO has been extended with a "receivers list" and a proposal to add/remove addresses from it.

However this list (actually a set in the implementation, for efficiency) is not practically used anywhere else in the contract.

We may consider removing it to save some costs.

Acceptance criteria

1. removing the "receivers list" from registryDAO is considered and discussed
2. if deemed acceptable, the changes are made to make such a removal

[pasqu4le]

Note: tagged as p1 for the discussion, but the priority for actually making the changes (if any) may go down.