[keyserver] Update usage of threads.default_role in role-deleters.js

Summary:
We need to get rid of usages of threads.default_role throughout the codebase so we can eventually drop the column.

This diff handles the usage in role-deleters.js

Existing code: Gets the default_role ID so we prevent it from being deleted (keyserver validation)

Depends on D9974

Addresses part of ENG-5833

Test Plan:

1. After updating the query, console.log the default role ID that is retrieved and make sure it corresponds to the correct role
2. Make sure that even if the client could delete Members, the keyserver still blocks it

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9975