[keyserver] Drop the default_role column from threads

Summary:
We have now been able to remove every instance of threads.default_role from SQL queries in the database, so it should now be safe to drop this column.

The stack up until this point should be fine to land as an 'intermediate' step before introducing the admin special role

Depends on D9981

Addresses ENG-5834

Test Plan: The migration succeeded and the column was dropped. I also simulated some normal behavior on Comm and made sure no parts of the app would crash

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9982