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 [[ https://linear.app/comm/issue/ENG-5834/drop-the-default-role-column-from-threads | ENG-5834 ]]